I was cherry-picking Maps, but I remember that not all iOS apps were updated to the iOS7 look and feel when it came out, it was only at point releases that the few that weren't updated received the update. If Apple has a new shiny Safari browser ready, they have to bundle it with the whole OS update instead of pushing an update on the App Store.

One could even say that the advantage of Google being able to system app updates on the Play Store is that Android apps competing with iOS ones (not a great comparison, but Google Now and Siri) have an advantage in that Google keeps on pushing updates regularly as and when they are ready, whereas Apple would update Siri during a major OS update or the incremental ones. [I am aware that back-end services/responses are updated regularly by both, but I'm referring to code which would change an app's behaviour].