alt Hacker NewsIn practice upgrading kernel can easily cause performance regressions and cause multiple other issues (reduced battery life) so there's a lot of risk for zero reward for an OEM to do that.
After all, they're on the hook for not breaking users already working devices and don't get anything by risking lawsuits and recalls.
I'll grant that changes leave the possibility of regressions, but that's true for minor patches too, so you already need a lab set up to catch those regressions, and if you've got a lab set up to catch regressions and engineers who can fix them, then you might as well take the bigger upgrades too.