My understanding is that kernel is part of an entire signed OS image. Since the kernel is part of a greater, immutable signed image, you can't just change a part of that image since it is cryptographically signed.
If you really want a newer kernel on the device, then I guess you would just have to make a custom ROM with a more up-to-date kernel. That or you can use such a custom ROM as if one is available.
At least this is my understanding of it.