It's not supposed to break userspace, but it does from time to time. There's a surprising amount of userspace code which has to check the kernel version number

It's not supposed to break userspace, but it does from time to time. There's a surprising amount of userspace code which has to check the kernel version number because some behaviour has no "feature check" that can be used. On rare occasions that gets tricky when a distro has backported some kernel change to an old kernel version. Because it's not supposed to be done, most breaking changes are unexpected so it wouldn't be possible to implement semver anyway, or are bug fixes which happen far too often for semver.