Chipset manufacturers ship close source drivers which makes it impossible to update the kernel to a major version. Huge third party roms like Lineage OS do not update the kernel even if you install a newer Android version.
ASOP maintainer resigned over this issue
Currently, there are a few efforts fix this issue such as
Collabera is spear heading the effort to run Android on Linux graphic pipeline
I.MX platform have upstream gpu drivers. I.MX6 is upstreamed and working
I.MX6 Development boards can boot Android
This I.MX6 5 in tablet should have full upstream support
In the future, Librem 5 will be the first fully open GNU phone.
I.MX8 gpu driver development is a work in progress
Anholt has been working on Videocore for Broadcom, but I find it a shame by the lack of adoption among handset manufacturers.