You're missing the point. You can have Tom.Hanks@twitter.com but you can't have Tom_Hanks@othertwitter.com
So when you come to the fediverse, instead of searching for Tom_Hanks@tomhanks.com, you just search for Tom_Hanks, and the fediverse will know that defaults to the account Tom_Hanks. Which is the same account on Lemmy, the same account on Peertube, the same account on pixelfed.
Because it's all Tom_Hanks.