> I don't understand why mobile systems do not attract OS builders.

They're graphical consumer devices, the quality bar is so high nobody can reach it except huge well funded teams. It's like asking why desktop Linux doesn't still attract OS builders, or for that matter, why the PC platform doesn't attract OS builders. Occasionally someone makes an OS that boots to a simple windowed GUI as a hobby, that's as far as it gets now.

A lot of these HN discussions dance around or ignore this point. When people demand the freedom to run whatever they want, they never give use cases that motivate this. Which OS do they want to dual boot? Some minor respin of Android with a few tweaks that doesn't disagree with Google on anything substantial (Google accepted a lot of PRs from GrapheneOS people).

Nobody is building a compelling new OS even on platforms that have fully documented drivers. There's no point. There are no new ideas, operating systems are mature, it's done, there's nothing to do there. Even Meta gave up on their XROS and that was at least for a new hardware profile. Google did bend over backwards to let people treat phones like they were PCs but it seems regular Android is in practice open enough for what people want to do.