I don't see how that's related to 'general computing'. Microsoft does not have a monopoly on the distribution of software.

That's exactly my point. They didn't in the past, but modern OS vendors do.

How so? Google only reviews apps published on their store. You can upload an APK anywhere, for anyone to download and use on their own device. iOS is obviously more locked down, but globally, Android has greater market share.

And that is just native apps. You can publish whatever app or game you want on your website, and the server for that site can be a simple Raspberry Pi in your home.

With the exception of IRC (and given the popularity of IRC that's not a huge exception) no one will use your chat app if it's not on the iPhone.