It doesn't work like that when you're one of two players in the segment, that's what antitrust is about. "It's their platform" would work well if there were any significant alternatives, which there aren't.
There are many popular app stores on Android and you're free to use any of them (or none of them, if you just download apps directly from websites) without rooting your phone, jailbreaking, etc.
From the perspective of an Android user, a lot of the alternatives are interesting. Especially F-Droid.
From the perspective of the app developer, it doesn't work. You use Apple and Google or you lose 95% of the market.
And the same constraint keeps it that way. You lose most of the Android market if you're not in Google Play, so nearly everything that isn't explicitly banned can be found in Google Play, so most users have no occasion to install any other app store and the friction to developers using another one to the exclusion of Google Play remains high.
I'd agree completely if you were talking about Apple and the App Store here, but this is Android. You can install alternative app stores on it without rooting it or any sort of advanced user skills. F-Droid is pretty great IMO.