To do so, there would need to be a centralized or federated list of all accounts. If it was centralized it would be against federation principles. If it was federated, then anyone who sets up a server would potentially have access to all email addresses. Perhaps,more in the style of email, like @user but that would likely lead to more confusion as people would put their email and it is essentially the same information.
I do agree that log in as the opening page is not a great experience. Helping people to find an instance should be easier to filter. Rather than a list by popularity, let them know choosing by interest or location can help find relevant content and have easy to use clickable filters to do so.