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.