Ask HN: How Are the Play Store and the AppStore Not Monopolies?

I keep reading horror stories about developers having their apps removed from these stores and I still fail to understand how they're not legally considered as monopolies largely abusing their dominant position...

I get that neither Android nor iOS are monopolies on the smartphone market, but why aren't Android and iOS themselves considered as markets of their own, on which the Play Store and the AppStore are monopolies?

Why don't they NEED to propose alternative stores on install as it was the case with browsers on Windows in the past?