The reason is that those CPUs have two types of cores, performance and efficiency, and only the former supports AVX512. Early on you could actually get access t

The reason is that those CPUs have two types of cores, performance and efficiency, and only the former supports AVX512. Early on you could actually get access to AVX512 if you disabled the efficiency cores, but they put a stop to that in later silicon revisions, IIRC with the justification that AVX512 didn't go through proper validation on those chips since it wasn't supposed to be used.