When you receive an invitation to a server, you're presented with a textbox that reads "What should everyone call you?" and you're unknowingly creating a new account. Then you're asked your birth date and then for your email. You type your email and it's already used, obviously.
By this point you don't want to go through the whole process of deleting your browser history to log into your existing account, so you go along with the new account thing and use another email address.
Before you know it, you have 5 different accounts and don't remember which ones you use for which servers.
Yes, there is a "use existing account" link, but it's not prominent, the "What should everyone call you?" textbox with the big "Continue" button are the only psychologically viable option unless you've already gone through the whole process of involuntarily creating many accounts.