Because it's a mobile platform and "GNU+Linux" is laughably terrible in this space.

It's almost like Android has put millions of expert dev hours into making it the most used OS in the world. Like GNU+linux on laptops only works the way it does because of android-upstreamed battery saver kernel features.

But a mobile is also people's most used devices with all of their data, bank accounts etc there - it has to be safe. And GNU+linux has not even a single thought about security, while android just has it worked out (every app runs as its own user, so it's even built on standard UNIX security).

A mobile OS also has to race to suspend and for that it needs cooperation from "apps" -- desktop apps just run, they don't care about anything besides SIGKILL. That's not a workable model on a mobile and android solves it.

And I say all that as someone who runs linux everywhere I can and I absolutely love it. It's imo the best kernel out there -- but the userspace is not where it should be and if anything, the correct question would be what can we take from Android and add to GNU+Linux. (And nix is fantastic, but it's a packaging solution, I don't really see how it comes into question here. I can run nix on my android phone just fine by the way)