To my mid/end game players, what keeps you playing?

Ive been playing this game on and off for the past like 6 years, but at this point its becoming much more off than on. I have gotten to the point where I have pretty much everything in the game Ive deemed useful in any way. I have gotten every prime frame and most of those which dont have prime varients, practically every mod in the game, and every weapon that has a usecase it excels in. I have done every form of content in the game, even doing self made "challenges" like going to level cap in VC. There is no "stronger" that I can become. I have the best weapons, I have the best frames, and Ive done the most difficult content the game can possibly offer. All of this has left me at master level 21. This seems crazy to me. Is all thats left to simply collect for collections sake? I love this game, and really want to keep playing it but I find it so hard when everything I feel like I have left to do is to just go farm for X weapon for Y frame that will preform 99% worse than my 2000p rivened torid, or my max built titania. If anyone else has experienced this feeling, or anything similar, please let me know if you found a drive to keep playing, and if so what it was, or if you simply realized that your time with Warframe is over.