How does Mozilla benefit from keeping it complicated? Or Microsoft, for that matter? Neither of them have a mobile App Store, so are they just playing along for the hell of it?
You control the browser, you control the ad experience, the use data, etc.
It's unclear to me what an App Store has to do with anything. (Microsoft does have an app store, by the way. https://apps.microsoft.com/home?hl=en-US&gl=US )