I've also asked repeatedly why those kind of projects are not using android forks instead of other things, and I never got an answer that made sense.
Android is open source and has proven that it's a robust system, I don't understand why people want to use something else.
Also, has anyone really done anything more than just tweak AOSP? Honest question, I don't know. If say Google completely abandoned the AOSP and went full Fuschia (I know this isn't likely), is there any individual or group out there that knows enough about AOSP to take it over?
Probably downstream vendors like Samsung have the technical capabilities. I seriously doubt that they'd be interested in continuing an open source project for wider use, but just maintaining their own fork would be totally doable.