It is a matter of responsibility. If you can log into any lemmy instance or mastodon server with the same account, then which server takes responsibility for your actions in the fediverse?
I have seen instances be defederate from because of their lax account creation requirements, or because of harrasment from users from a specific instance.
If an account can log into any instance, then who is responsible for banning the account?
Just like orbiting something. Satellites are constantly just falling back to earth, but with enough grace to always miss earth. I bet satellites would be great cyclists!