It is, and it's being actively developed by Google at a rather big expense.

Just like pretty much every other major opensource project - even Linux itself is majorly developed by a few corporations.

Actually, it's now a few but rathet a pretty big bunch of very different corporations & many of them direct competitors. And thanks to the kernel project & its license, they can still work together towards the common good.

I'm sure it's a large expense. However, they make more money from the data and market share they get from it.

Why else would they keep moving things out of AOSP and into their proprietary frameworks?

Same for Linux itself - however in that case, there isn't incentive to make it proprietary, because the companies are either making money from hardware sales or from support contracts.

Yup. Used to be called "Google Experience", even. I don't see how this proprietary part of "Android" or whatever can exist outside Google as a business. And AOSP sans Google Experience is a thing already, as well.