There are some people in other networks who feel very strongly their answer is the right answer to the great question of decentralization [insert south park atheist otters]. I think they are in part frustrated that ATProto (not the "right" answer) has attracted the users and developer. The meanness and lack of curiosity certainly provide the undertones to justify this interpretation.

Because centralization matters. It is what stops a hostile agent from ruining things. There is no real win in being "semi-decentralized".

Purists. There are some people who run email for their personal domains on Raspberry Pi machines sitting in their homes. Maybe they want everyone to live the same way?

Personally, I think it's better that there is choice. I do not want to run my own social media site any more than I wanted to run an IRC server.