Help me understand the appeal of endgame

TL;DR: I hate that your equipment (and therefore how rich you are) virtually becomes the only factor you can and must improve in order to "progress" through the endgame. All of your build's subtlety gets dumbed down to a contest of big numbers where you must deplete the mobs' HP even more instantly than they deplete yours. I don't get the appeal at all, and I can only hope that someone can point to me where and how my understanding is flawed.

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Allow me to preface with a bit of context.

I started playing in 0.1.0, put almost 600hrs on and off in the game (didn't play in 0.2.0) and brought multiple characters to the endgame. However, I have trouble staying motivated past the campaign, pretty much the only thing that keeps me going is FOMO because all the discussion about the game is centered around endgame, I see so many people say the point of the game is the endgame, and some go as far as wanting to start there right away, which absolutely boggles my mind that anyone would even suggest that.

I also feel it's important to mention that I have never played PoE1, all I know is that there's lots of stuff that was brought back directly from it with the implications that most players know how it works, and with little to no explanation for newer players like me. I also don't watch any PoE streamers or anyone more knowledgeable that I could try to blindly copy until I figure out what the hell I'm doing. As such, I'm going to give my current perception of it, and make a lot of assumptions and say a lot of most likely incorrect things in this post, so all I ask is that you bear with it and explain to me what I'm getting wrong or missing completely.

Also, I know there's nothing wrong with enjoying the game for the campaign only or however else you want, but please don't give me that. I'm making this post precisely because I WANT to "get" the endgame. It doesn't matter whether I actually end up liking it or not, I just want to be able to make a proper opinion of it for the right reasons by having all the cards in my hand.

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So, with that in mind, I'll just start blasting. Sorry in advance.

I feel like the further you progress in the endgame, the more your actual build becomes irrelevant. Even right as you get from campaign to mapping, the equipment you had suddenly becomes obsolete, even if you were wiping the floor with everything up to that point. And from hereon, the only "real" progression you can make is upgrading your equipment's price range from exalts, to tens of exalts, to divines, to tens of divines, etc..

But that in itself wouldn't be a problem if it was only one of multiple ways to upgrade your build. And technically it is, but all the other methods end up falling short one way or another, in ways that simply don't apply to equipment :

• Support gems and skill levels are significant, but unlike equipment you can choose what to get from uncut gems, which means that from a statistical standpoint, it's virtually impossible to hit a roadblock from those. You will end up capping and increasing all the juice you can squeeze out of them pretty organically as you progress through the campaign, get higher gem tiers and unlock more slots for support gems. You get enough uncut gems to afford spending some on experiments as well. By the time you reach endgame, your gems will be pretty much set in stone (hah), and you can hardly make significant upgrades by messing around with them anymore. I suppose the introduction of lineage gems is there to spice things up on that end, but even then they feel like Tier 6 gems, and thus can't hold a candle to the equipment power creep.
• I really enjoy optimizing my passive tree, but a lot of nodes are about attack and/or defense, which directly scale off of your equipment, and most increases offer "diminishing" returns in the sense that +10% from base total is 10% more, but +10% from 200% total is only 5% more. This means that upgrading the base is paramount here as well, and that you're better off favoring yields that are agnostic towards your equipment like skill duration, limit increase or major nodes like Chaos Inoculation, Resonance, etc., and leave the leftovers to such increases as a bonus. And by the time you got all the ones you want, you won't have much points left to split between attack, defense and extra utility relevant to endgame like stun threshold or Chill/curse mitigation.
• Ascendancies are a mix of both skill gems and passives, so they have the exact same strengths and weaknesses, only dialed up to 11.

There's one thing that all of these have in common: opportunity cost. The resources to make use of them aren't hard to come by (for the most part), but the ones you end up choosing matter just as much as the ones you don't, which encourages theoricrafting and ingenuity, and is something I love delving into.

However, there's no such thing in regards to equipment, there's only... cost. By the time you get to the endgame, you'll have squeezed the vast majority of what you could get out of the other avenues of upgrades. But with equipment, not only you'll most likely not even be remotely close to how far it can take you, but as long as you have the currency for it, there is no such thing as opportunity cost, you can have it all for the right price.

And because of this, I would argue that equipment isn't part of your build, because there's no real choice to be made, only an objectively correct answer to what you should equip in any given slot that you must try to replicate as closely as possible with how much you're willing to pay. Even if you argue that you only have 6 modifiers per equipment, getting to the point where THAT is your main concern is already a luxury far beyond the means of your average player. And no, the type of defense you're going with doesn't count when the choice is already made for you by the stats your weapon and skills demand. Ironically enough, Uniques also fall victim to this phenomenon, with the vast majority being 1ex garbage and a select few being so overtuned that they can fit in virtually any build and thus have entry costs measured in divs.

Thing is, requiring no thought process for your equipment is actually fine by me, the other options should be plenty enough to keep you busy... Except its power ceiling is so far above everything else that your entire build can be made "viable" by simply spending millions on it. Which completely invalidates any effort in making a coherent build in the first place. If all I need is an AoE skill for clearing, a DPS skill for bossing (optional), and a godlike weapon (+ gloves & jewelry, but I'm keeping it simple) to make numbers go brrrrr, what's even the point of putting any thought into it??

Here's the thing: on paper, a well-made build might not need as expensive a weapon to work, but the entire process of buying or crafting (or looting, if you're that guy) a "good enough" weapon is still entirely reliant on RNG, and the power gap between a white weapon and a full T1 weapon is so wide that you can never tell exactly how much power you need, you just know that you must upgrade. And since your weapon matters so much more than anything than anything you can scrape here and there, this means that an unrefined build ends up at the exact same level of progress (and cost) as a refined one, which makes refinement a complete waste of time. And speaking of costs, even the price of the upgrade itself is ultimately up to luck, in case there wasn't enough reason to forego optimizing. Veterans can even skip that process altogether by getting hand-me-downs from their previous characters in that league and that will most likely be way overkill.

For any way you can think of to challenge any given build, it's so easy to "cheat" your way through that it's pointless to objectively compare builds -- just get the most expensive gear you can afford at any given time and hit things until you get bored, no need to test, fine-tune or rethink anything in your build because even if you perfect it, money will always have the last word on it eventually. Because nothing else can keep up.

I don't remember where I heard the phrase "Hell is when you get everything you ever wanted at all times" or something of that effect, but I think it's particularly relevant here.

And who knows, maybe this issue would be made completely moot by well-adjusted difficulty. But the further you go, the more the game turns into a high noon shoot-out between you and the mobs where the first to oneshot the other wins. It completely removes any notion of struggle, because struggling means instant death by that point, and that's as much of a struggle in my book as trying to find a clever comeback to a pigeon shitting all over your face. Unless of course, you're specifically going for a sustain build with leech or Tecrod shenanigans -- in which case nothing short of the Abyss degen aura can even hope of reminding you that you were mortal once upon a time, which loops back to my complaint of removal of struggle either way. Not to mention, this instakill-or-be-instakilled approach goes completely against the more gritty, more "Souls-like" combat system the game devs said they are aiming for, basically forcing you to play a FromSoft title as if you were allergic to Vigor.

Ironically the campaign, while technically easier than mapping by every conceivable metric, feels to me like a much more compelling and rewarding struggle. Even when I end up oneshotting enemies, I know I don't have to in order to win, and that just means I'm doing something right with my build even at such an early stage. But how can I be so sure of it you ask, when I just spent the last 1000 words complaining about builds being irrelevant in endgame? Because at no point during the campaign do I go out of my way to craft or look for better gear than what I naturally find or buy off of vendors, since I know I can easily replace it with a better base in just a couple levels. Achieving satisfying power levels despite using subpar equipment feels awesome, because it's a direct result of all my thought process with my skills, my support gems and my tree pathing. And not just that, but the sense of progression you get from acquiring new skills, support gems and even equipment, trying things out, feeling the impact of each step towards your goal, is something that is (almost) completely lost upon reaching the endgame. Which obviously has to happen at some point and thus shouldn't be a problem, if only the game could preserve, let alone improve on the levels of struggle the campaign is built on, and not give it a solution as automatic, predictable and boring as "just buy better gear 5head"...

...Which is why I cannot wrap my head around the idea of people wanting to fast-forward or skip the campaign altogether. And so here I am, making this post, wondering if I'm missing or misunderstanding about the endgame and its progression that makes so many people more interested in it than the campaign, if it's something that requires you to have played PoE1 to fully appreciate, or if it's just my expectations that are misplaced to begin with. Again, I'm not looking for a "it's just not for you" type of answer, I want to properly grasp why and I can only hope doing so will make something click in my head, and if I still can't relate by the end of it, then at least I'll be able to say that I really tried.