Because manufacturing mobile phones is hard and expensive. Companies have large teams of hardware and software people will incentivized to develop phones and lots of them fail.
The best we’ve got so far are exactly custom android builds like grapheneOS or lineage that replace software on phones other companies build. Apparently google doesn’t like that, though, so they’re going to continue to make AOSP worse and worse to protect their spyware.
The best case currently probably varies between something like grapheneOS on a pixel phone or lineageOS which is less privacy focused but has a much wider range of supported devices.