The problem with these 100% vertically integrated stacks is that every hardware release could be completely different and it'll take years to catch up with just that release. In the intervening time more hardware generations were released - It's been 18 months since the M1 release. Asahi doesn't have 3D acceleration, video encode/decode acceleration, or support for many of the things that make Apple Silicon any good (i.e. the fixed function / low power consumption hardware for the majority of user tasks). At this rate it's going to be years before it's "done" and we already have a successor generation of hardware.
I'll leave you with a quote from the Asahi docs
> Development for an undocumented platform is a treadmill of work. Every new feature requires reverse engineering the relevant hardware, writing drivers, testing those drivers, then getting them upstreamed. Even after a driver is upstreamed, maintenance and optimisation is sometimes required, for example if Apple introduce a breaking change to any firmware we are required to interface with. For developers the work is never really done
It's the same reason we don't have third-party images for most Android phones that are anything beyond tweaks of existing Android images.