Short answer: People spend what they want to spend.
Long Answer: About 2-3 years ago, there were way more members and more TBT to circulate around. People made ACNL shops, art shops, signature shops, or just resold collectibles. The chocolate cakes used to cost over 2k TBT around the time I first joined. The longer period we spend without there being a new main AC game, the more members would leave or be inactive. Many of those members had rarer collectibles like Pokeballs so nowadays, there's a much smaller amount of people who have them.
In order to get collectibles, the best way is to participate in forum events (there's an event every few months [the next major one will probably be the Fair]) and then resell. There's almost always a limited amount of most collectibles. Every now and then, the forum may do restocks which always attracts competition. Despite the competition, it's not denial-able that so many collectibles have actually gone down in value. The more a collectible is restocked, it generally goes down in value (i.e. chocolate cake, fruits, light blue house).
Most people get collectibles for either a) they look cool/cute/or bragging rights, b) to resell, or c) to gift them to others
Either way, it's fun to get these cute little pixels and make the forum a little more unique. Hope this info helps!