There's a choice of app store in Android and almost nobody uses Amazon App Store, F-Droid, Samsung App Store and so on. It's great that they can exist, but I don't think the Mac App Store (which has been neglected since day 1) offers any guidance.
People mostly use what they know or are giving by default. In desktop, be it Windows, Linux or macOS, people aren't used to app stores.
It's a shame because the old Fire tablets were pretty good for kids, but now the malignant ad and micropayment cancer has returned so it's no good anymore.
You also can't install the various alternate stores themselves through Google Play and Google purposely makes installing APKs a pain for regular people.
And then it's "see, nobody wants them" after they purposely put a wall in front of them which most people can't get past.
So developers still need to be in Google Play to get all the users who can't figure it out, but if everything is in Google Play then even the customers who could figure it out have no incentive to go through the trouble.