I've always wondered why Google, seeing the problem of manufacturers diluting the "pure" Android experience and turning it into 50 shades of grey, doesn't clamp

I've always wondered why Google, seeing the problem of manufacturers diluting the "pure" Android experience and turning it into 50 shades of grey, doesn't clamp down on the terms of usage and force the phone mfgs to provide a single user experience, like Apple excels in. They are ultimately holding all the cards. Don't like our terms of service? Go license Nokia's OS.