In short it's hard to do right.

It would have to be in a way that doesn't chain you to one central identification server that could ban everyone, but also still handles moderation actions properly.

People would probably also want their identity to be portable (movable to other source instances) which will mean different things to different fediverse services in terms of what would move, and things like handling username collisions. We can't even lift and shift between instances of the same service yet, whete it should be a simple one to one.

Most of all, a huge part of the conceptual point of the fediverse is that you should be able to interact with the rest of it from any point inside it (with exceptions for intentional defederation between some servers). On a conceptual level, you shouldn't need to create a login for mastodon when you can post to mastodon from lemmy, and vice versa. You create your login on the type of fediverse system you want the interface of, on a server instance you agree with the moderation policies of, and then you interact with whatever you want from there.

In practice it's not so simple, but that's largely seen by the various devs as a software interoperability thing, not a sort of single sign on identity thing.