So fork AOSP today. You actually have a working stack, working systems (Lineage, GrapheneOS, /e/OS), and it's far less work than bringing mobile Linux up to the level of AOSP.

But no, let's reinvent the wheel and start the complete ecosystem from scratch. Of course, looking at the history of Linux, this is not surprising at all. Even something immature as mobile Linux already has multiple projects + stacks (Sailfish, ubports, Plasma Mobile, PureOS, etc.) trying to achieve the same. I say this as a long time Linux user (since 1994), the Linux desktop community manages to shoot itself in the foot over and over again.