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.
I’m not familiar with every client, but on mine it only hides the domain for users on my own server. (Early email used to work exactly the same—you could send an email addressed to just a username with no tld and it would go to the user with that name on your own server by default.)
I’m not using any client. I’m just using the browser that came with my cell phone.