Updating kernel would require updating the AOSP binaries, too, but getting Sailfish to support it is a lot of work, even for minor updates. It would also require the users to flash the new AOSP themselves, which isn’t currently well documented, nor is it “a quick and simple update” to so. The newer AOSP package also bumps Android major version, so it’s double the hassle, likely even worse than that.
TL;DR One doesn’t simply update the kernel