Forking is all easy, keeping it up to date year after year as codebases diverge is a whole different story.

I could see Samsung doing it. But they won't, they're too good buddies with Google. But they have the resources. A Motorola no. The grapheneos team won't either, maintaining a disparate fork and introducing new features independently from aosp would just be beyond their scope. You're not just hardening at that point. You're basically doing everything.

Don't forget when Huawei didn't fork. Well they started with that but then replaced every component with their own design. It's easier because if you fork you're still bound by decisions made by the original party. Better to greenfield the whole thing then.

And look at how many people made a soft fork of chrome with some ui changes. There's tons of those. There's no hard fork that no longer follows Google. Even a large company like Microsoft didn't.