Why do MMO players gravitate towards the server with the most players? I don't play MMOs but I'm guessing it's because they can only play with users on the same

Why do MMO players gravitate towards the server with the most players? I don't play MMOs but I'm guessing it's because they can only play with users on the same server as them. Federated services don't have this limitation. As a counter example, if the behavior you described really did happen in federated systems then why hasn't it happened to email?