Yeah but realistically that'd just return: ERROR: Unknown column 'submissions' or whatever because you'd probably have a separate submission table. Still, you're right that this simple stuff is what SQL is good at.
But having all that data in an in-memory hash is way more flexible and convenient. He can do any arbitrary operations on it, not just the stuff SQL and stored procedures are good at.
select username, karma, submissions from users where submissions > 10 order by karma desc
but you don't see me running an uber-hip VC firm, do you?