Ask HN: Why doesn't Microsoft build its own Android “fork”?

Why doesn't Microsoft just build a custom version of Android? They get instant access to the vast android apps and they can integrate it deeper with the Windows ecosystem and also innovate in their own "fork" of android.

They could eventually surpass other versions of Android and become the goto android implementation.

I never understood why they didn't go that route - after all Android is open source. What am I missing?