What if it's just not the kind of thing normal people actually care about? I know that's a taboo thought here on HN, but a well-maintained walled garden is often just a lot nicer for users than too much freedom.
Steam, for example, still dominates PC gaming despite the Epic Game Store, the Microsoft Store, GOG, etc. Conversely, I still don't trust most of the apps on the Google Play Store. And the macOS desktop app marketplace is such a mess, and I wish were more apps were just available on the official Apple app store instead of requiring some third-party purchase pathway (and usually delivered with a DMG image or some Homebrew cask, with no verification or developer signing at all).
Even if Apple allowed third-party app stores, I wouldn't want to install them (nor handle billing through Epic instead of Apple, especially now that Epic is half owned by Tencent).
The peace of mind of having to trust just one marketplace, and also the convenience of being able to easily sync my purchases, save files, etc. across multiple devices using a single login is way more important.
It's not just the technical limitations, but also that some people just don't actually want the wild west when it comes to their computing devices...