Last I checked the App store was not a significant revenue source for Apple. My guess is the review process is expensive and most apps are a net negative from Apples perspective.
One way around this would be if apple reserved a portion of sales, but with the number of 'free' apps out there this would probably not work. And for near breakeven apps it would probably result in similar numbers. (EX: 100% of the first 10k, 10% after that.)