Why are parts so hard to find? (Newbie)

I want to make a complete MWS 11.5" URG-I upper receiver as a quick-swap for a 14.5" that I got because it was out of stock, but even the parts (handguards and receivers too) are out of stock at almost all of the retailers besides the ones that I've read on here and HRC that have terrible shipping and customer service.

I've only gotten back into things a month prior and when I did play almost a decade ago I always rented equipment so I mostly have 0 idea what I'm doing or how difficult it was back in the day, but why is stuff so hard to find right now? Especially in the US.

Just statistically I would assume that there's not that many people interested in or playing airsoft enough in general and then to own a GBB & go through everything that comes with that is an even smaller subset of people. Shouldn't that mean that there wouldn't be a ton of people buy up all of the stuff I need? Is supply really that low conversely because there isn't that much demand? Just a bit frustrated.