> What's wrong with pixel drivers for instance? I'm of course not talking about those written by e-waste ODMs like Tinno who can't even release GPL kernel sources properly

A couple of issues , back from my sailfish os porting days (haven't touched it in half a decade so things might have changed):

1. You would still need to run a lot of Android user space to make use of these drivers. We had a separate second init system that runs a stripped down version of Android init, starting enough services to make use of these drivers.

2. These binary drivers are tied to the kernel and Android versions that were released with the phone. So if you wanted some of the newer kernel features (back then it was like better container support), you're out of luck.

Usually, all this means you rely on a lot of hacks to fix some bugs... And porting takes a lot of time and effort.