A few reasons. Lack of centrally controlled algorithms controlling everyone’s feeds is a big one. That is perhaps the biggest reason. Also, there are different incentives here for creators and even just users in general. You’re not posting to please an algo, but rather to make connections with people and express your ideas/art/etc for them. You’re rewarded or “punished” so to speak by other people using the network instead of by algos and policies decided to maximize ad revenue, or worse to drive some political agenda.