It's a combination of things - they feel like freedums are going away so they want to defend it to the death/cold dead hands. Right now there's a persistent ammo shortage - makes everything more expensive. I did however notice that 1,000 rounds of 9mm were going for $334 - I spent a second considering it. But I can make ammo/have it made for me so it's less of a concern.
I will say that for some people...they legitimately just like to build guns. They buy the upper and lower, get the kits, and then mod it out however they like and then see the quality of craftsmanship once it's tested. It's a hobby - some I know don't even shoot them (really weird but alright).