There is no centralized authority on Twitter either, because you can always go and use Facebook. The Web is a federated system where everybody just decided they don’t want to talk to anybody else.
If you make a Mastodon account your digital identity is bound to that one server. You can’t move to another server. You can’t communicate with other servers that got defederated. Exactly the same as Facebook and Twitter. It’s only decentralized up until server admins decide that it isn’t, which already has happened numerous times in the past. The whole thing is basically just based around wishful thinking. If everybody would be niche to each other and servers would run forever, it would be totally fine, but that’s not how the world works.
There are many entities that are part of a federated system, just like email
Email is a terrible protocol by modern standards and the problems of federation show in email pretty clearly, as the majority of people will stick to Gmail and a handful of other major providers. There is no reason to repeat past mistakes. The saving grace with email is that you don’t have the moral police looking through your emails and kicking you from their server when they find something they don’t like (outside of sending spam), with Mastodon on the other side they do exactly that.