My stab: A Mac Studio will have 400 GB/s or 800 GB/s of memory bandwidth. Not that you can't get there on x86 e.g. a 12 channel Epyc Genoa setup can do 460 GB/s or 920 GB/s total when doubled up but now you're talking about buying 2 latest gen Epycs and 24 high speed dims to get the raw bandwidth back all while ignoring the access is a bit different.
Curious to see if there were other reasons.