x86 architecture is standardized enough that you can use a general x86 kernel on any processor. Like with OM, the x86 version works perfectly fine on the newer AMD processors. AMD and Intel have gone in slightly different directions, so that is there the znver1 kernel comes in with optimizations for Ryzen chips.
ARM is a different story. Qualcomm’s chips are vastly different from Apple. Qualcomm even uses two vastly different ARM builds because they bought a firm that had a design, while already having their own design in place. That is what the ARM vs. Qualcomm lawsuits are about. So, building a kernel against all of these designs can be time consuming, especially for the Mac, because their drivers are not pushed to the kernel.