Of course the argument for this is "why are iOS users more inclined to purchase games and IAPs compared to Android users?". I'm legitimately curious about this, but my first guess is these two factors:
A. iOS is generally more expensive and thus users are more likely to have disposable income
B. The IAP system means the only barrier to purchasing stuff on a completely new app is performing touch/face ID
Don't forget that Android has more than one store and depending on the company you buy your phone from it might even come with another store pre-installed, such as the Samsung store.
So comparing iOS store to only the play store will ignore a section of Android users.
A. iOS is generally more expensive and thus users are more likely to have disposable income
B. The IAP system means the only barrier to purchasing stuff on a completely new app is performing touch/face ID