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.
Like, say, if esteemed Academy Award nominated character actress Margot Robbie was on social media, it’d be utterly brilliant to choose to be on Lemmy, as no-one would believe it even if she went by her real name.
Personally I would find that highly amusing, a testament to having the soul of a true thespian.