I'm not certain there is a direct correlation between the apps that make the most money in the App Store, and the apps that people spend the most time in.
It would mean that all the built in apps, and all the free apps in the App Store relatively unused, and that advertising models are a fools errand as no-one spends enough time in the apps. I suspect companies persuing these kinds of apps, including apple, are seeing enough usage to make a business case for their actions.
Of the free apps in the US store right now are keyboards (new for iOS 8 and don't really count as "apps") and Facebook Messenger, iTunes U, Amazing Thief (game), The Maze Runner (game), Instagram, Facebook, Beach Buggy Racing (Game), etc.
Games are extraordinarily popular on iOS. I see people gaming constantly on planes, trains, busses, etc.
It would mean that all the built in apps, and all the free apps in the App Store relatively unused, and that advertising models are a fools errand as no-one spends enough time in the apps. I suspect companies persuing these kinds of apps, including apple, are seeing enough usage to make a business case for their actions.