The more refined apps (which are the ones customers care about) use a variety of Android platform services, not just the barebones stuff -- homescreen widgets, rich notifications, bundled notifications, app-defined quick settings, fingerprint authentication, Vulkan API, Cast support, Material themes, Play services etc. etc. etc. Building a complete compatibility layer means pretty much reimplementing all Android userspace.
Microsoft did something similar with the Astoria project, but they gave up on it, probably because of the Play Services dependency of most apps. And yes, you could enable it in Insider builds, and it mostly worked. Slap microG on it and you'll see it can be done.
