The average time each user spends using their Android device(s), and those users' average affluence, is so incredibly low that they still only manage to make up 50% of Google's ad revenue on mobile.

iOS users are way more likely to be (relatively) wealthy, and they tend to use their devices more (if you've suffered through using anything other than a top-end Android device from the last couple of OS versions, you can guess what one of several reasons for this may be).