Because the kernel developers aren't interested in assuming the maintenance burden of maintaining (possibly many incompatible and versioned) interfaces purely for the benefit of users who maintain out of tree drivers.

I'm surprised driver interfaces are still changing so frequently. Ten years ago perhaps, but they're not largely sorted today twenty five years+ after Linux debut?

There have been general architecture changes. Look at DRM as a better way to do rendering versus the old Unix way, for example.

Sure, but isn't that ten years old already? It's not like these big changes come every month.

Not "can't," but "won't." It's not considered worth the effort; the technical benefits are too great when doing it the current way. The political side effects are a great bonus but not the main motive.