Because let’s say you’re Tom Hanks. And you get TomHanks@Lemmy.World
Well, what’s stopping someone else from adopting TomHanks@Lemm.ee?
And some platforms minimize the text size of platform, or hide it entirely. So you just might see TomHanks, and think it’s him. But it’s actually a 7 year old Chinese boy with a broken leg in Arizona.
Because anyone can grab the same name, on a different platform.
That’s always the case, even for centralized platforms. Usernames are just usernames. Same thing with email. This is a fundamental problem with the internet and the solution is that celebrities and such host their own ActivityPub server (just like their own email server) or make it clear on their personal website what their own official account is somewhere else.