On Android the experience of manually installing an App is better than the average Windows installer. You download an apk, open it, click on install, wait, click on done. If it's your first time you get redirected to a settings page where you have to flip a clearly labeled switch before you can proceed. Better than the Windows experience of clicking "next" way too often while getting tricked into the bundled McAfee trial.

Yet most Android apps are only available over the Play store. It's a classic platform effect: all the customers are on the Play Store, so all the Apps are the, so all the customers are there. Windows never managed to get this kickstarted (not for lack of trying).

For app categories that aren't allowed on the Play store providing the apk works and it doesn't seem to be too much to ask of people. But if your competitor is on the Play store and you aren't then you lose on convenience. And of course the mobile app pattern of updating your app every other day only really works with the auto-updates of app stores.

I'm not sure I can fault Google for any of that. We can fault them for preventing other App stores from gaining traction by withholding features from them in the past, but the decision to use an App store at all isn't forced on anyone (on Android, iPhones are a different story)