One reason, I guess it's not possible because it's a complex OS?
But is the real obstacle being smartphone brands not publishing their hardware drivers?
It is so easy to install linux on a PC, yet I don't see the same happening for android while it's actually running a linux kernel, so it really begs the question.
It comes with optional sandboxed Google Play Services and Store, meaning that these run just like any other app, with no special permissions. You can give them only Network access. The Play Store is still the most secure way to download everyday apps, so a lot of GrapheneOS users use Google's Play Store with a burner account in a separate profile, usually the Owner (the main) profile - since you can then disable apps in Owner and install them into other profiles. And the sandboxed google stuff can be used to run proper Google apps without any problem. Even sandboxed Android Auto works.
It's not particularly difficult -- see Graphene and Lineage. The main issue is that there are few phones on which to run these custom builds. Ironically, Google Pixels allow to run other operating systems than the one they come with (the bootloader can be unlocked). Other than the Pixel and a couple of Chinese models, you are looking at low-end or ancient hardware. You can't just build a phone without OS and install Linux/Android like you would on a PC.
Android is open source, does that mean brands don't release the drivers, or the scripts involved in making the images?
Are those drivers and scripts proprietary?