

Linux phones try to build from upstream Linux, and the major phone SoC vendors HATE upstreaming their code.
They believe every character in their source code is absolutely top secret.
A middle ground I wish was considered more is taking Google’s kernel and the vendors DLKM partition/DTB/DTBO for hardware support, and putting a GNU userspace on top.
This has had problems in the past, because vendors would modify syscall tables such that they don’t match userspace anymore, but with GKI, I think we’re closer to that being a possibility

At least with the vendors I’m referring to (2/3 that make all Android phones), they just took the open source code, hacked it up as quickly as possible to get some basic drivers working, and moved on.
There wasn’t any “special sauce” in the source, they just didn’t want to spend the effort to upstream it
Edit: Just because you said “hardware open source” I wasn’t advocating for open hardware, just for hardware vendors to, ya know, support the hardware