Social networks are inherently monopolistic because people follow the crowd, and federation is meant to counteract that tendency toward userbase consolidation.

It's just that network effects are real. I want to be where all the smart, fun people are, not on a ded site or nazi island. I don't really care about who's on my instance, just who's on the network. Federation and decentralization were never huge selling points for me. And as Mastodon demonstrated, most people just want to log onto a big network and don't care or want to think about instances.