Why not? GrapheneOS and others show that it is possible to make viable operating systems on top of AOSP, which also have their own useful extensions.

It seems like a waste not to use an existing, well-developed, hardened, open source base, that at the same time provides great compatibility with most existing apps.

Since it is open source, it would always be possible to fork if AOSP goes off the rails.

I think the primary issue is that it is currently hard to get embargoed security patches, unless you have some partnership with an OEM.