Lots of Android devices are sold.

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).