Think about it like this...

It's the best hand. Given a confrontation against 700 other hands at the same time that aren't AA, AA will win most of the showdowns. The real question becomes "How long do I wait for my profit?"

The point I'm making is that the more hands that are in with AA, the longer it takes for the positive expected value to reveal. So if everytime you were all in with AA you were heads up, profits would be immediate and apparent. Your bankroll would probably shoot straight into the atmosphere. If every time you were all in with AA you were 7 handed, then enourmous swings would take place where you would come out ahead in 83 years. I'm being overdramatic, but you understand my point. The more hands that are in the pot, the more it stretches the +EV over a longer period of time. It is never negative however.