A main reason that Microsoft would want to retain their own OS instead of integrating with Android is to simply stay different. It allows them to be much more flexible with what they do. By just using Android, they are becoming a hardware based company - at least as far as phones go. Microsoft doesn't really want to do that. Instead, they're focusing more on the software and OS side of the coin.
An excellent example is Windows Continuum. This is a feature that would be significantly more difficult in the Android ecosystem...and almost impossible to do as seamlessly.
A main reason that Microsoft would want to retain their own OS instead of integrating with Android is to simply stay different. It allows them to be much more flexible with what they do. By just using Android, they are becoming a hardware based company - at least as far as phones go. Microsoft doesn't really want to do that. Instead, they're focusing more on the software and OS side of the coin.
An excellent example is Windows Continuum. This is a feature that would be significantly more difficult in the Android ecosystem...and almost impossible to do as seamlessly.