using the email address analogy - every server maintains its users. You can't log in to Gmail with your yahoo email either, but you can still email a yahoo user.

Having multiple servers that you can sign up for helps keep things decentralised. If all the logins are centralised, then either one of two things need to happen:

1. One single entity controls all the logins. And if that entity decides to go on a power trip (say... a completely fictitious example where he decides to start charging all servers a ridiculous monthly fee to use the login, then gaslights people who call him out, and doubles down when presented with call logs that show otherwise), the fediverse is dead.

2. Everyone has to keep a copy of the same userbase. When 1 person signs up on 1 server, every server needs to acknowledge that signup. This is going to create massive problems if/when the fediverse becomes huge - imagine thousands of people trying to sign up across thousands of servers.