The iPhone, iPod touch, and iPad currently come pre-installed with range of apps including Messages, Calendar, Photos, Camera, Clock, Maps, Videos, Notes, Reminders, Game Center, Newsstand, iTunes Store, App Store, Settings, Mail, Safari, and music. None of those apps can be downloaded from the App Store, nor can they be deleted. They simply are. But what if that changed? What if Apple still pre-installed those same apps, but updated them via the App Store?
Perhaps Apple can already update built-in apps outside of firmware updates, perhaps not. To date they haven't taken advantage of any such functionality. They haven't pushed out a new version of just Safari to patch a vulnerability or exploit. They haven't pushed out a new version of mail to add a clever new feature. They haven't pushed out a new version of Game Center to remove those super-annoying "[Your soon-to-be-former-friend] has challenged you to [scummy app you'll now be avoiding]." When there's a long gap between iOS updates, there's a long gap between built-in iOS app updates.