Because let’s say you’re Tom Hanks. And you get [email protected]
Well, what’s stopping someone else from adopting [email protected]?
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 why she hosts her own domain, instead of sending half a million followers to some random fediverse instance.