What for? Every kernel release is equal as far as maintainers are concerned, the list of features that go in depends on what is ready in time. They would have to change release processes significantly to accommodate "major versions", and I don't think they would want to do that.
Also, the kernel doesn't really have an external kernel API, the user API is supposed to be 100% stable, and its internal kAPI is nobody else's business, so there's one less reason for semver.