Because Intel/AMD regularly contribute kernel changes to maintain support for their own hardware, whereas Apple keeps making undocumented changes that Asahi has to reverse engineer.
I've definitely ran older kernels of Linux on new Intel/AMD CPUs where the kernel release vastly pre-date the CPU release.
I don't think that's it, as we usually don't even have to update the kernel: when I get a new PC, my old software still boots and runs. The answer has to also provide some analogous note that, unlike new x86 hardware having an interest in still being able to run old versions of Windows, new Apple hardware (maybe... one must presume for the story to be consistent) must not really care about being able to boot old copies of macOS.