Because its existence relies on a good will of Google. See:
Google will allow only apps from verified developers to be installed on Android (9to5google.com)
https://news.ycombinator.com/item?id=45017028
and
GrapheneOS accessed Android security patches but not allowed to publish sources (grapheneos.social)
https://news.ycombinator.com/item?id=45208925
> Any large enough entity can fork.
Only megacorps will likely be able to support a hard fork for such a large codebase.
> Hundreds already did, successfully.
Which of them are hard forks? China will not be a benevolent dictator of AOSP
> Fairphone
It's Android again.
There are indeed non-Android alternatives, but not in Europe. I use Librem 5 btw.