Advertising is bought because it works. Advertising in nice places costs more. iOS users are a much, much more attractive demographic than the general public who see/click ads.

A reasonably high-end display advertiser will pay a relatively high CPM to have their brand associated with an object like an iPhone with a high perceived value, even if no one ever clicks, and especially if theirs is the only ad on the screen at a time.

With the insanely fine-grained targeting information that Apple has about you and your device ---such as your name, age, country and home billing zipcode, how much you use your iPhone, your purchase history with Apple, whatever other information you've ever given Apple like income and occupation, your current location (in principle), what apps you like, what music you like, etc.---they can probably pick the right ad for each user well enough get really, really good average click through rates. You'd be amazed at how well you can crank up the average click likelihood even with way coarser targeting information.