I don't think it actually matters if Android or iOS has more devices. It is that there are so few players, so regulators care more. I don't really know much of anything about how the fee structures work for Android, XBox, or Playstation, but I know for the devices I own (and I happen to have at least one of each), I don't have to buy my games through the stores. Does Sony still get a cut of a game that I buy on a disc? To be honest, I also don't buy many games for any systems any more.
I know for Android, I've purchased the occasional app from the Amazon Android store, because it was on sale or there was some promotion. I also don't know if all in-app purchases need to be purchased through the official store(s).
EDIT: I see that Google does require in-app purchases from apps bought from within the app store to be made through the Play Store program, with the 30% fee. I also see that Google dropped Fortnite from their store, and Epic has sued. Epic says that apps purchased from outside the Playstore are disadvantaged in some way, so they don't see that as a fully viable approach.