I don't think there is any reason a PS box couldn't be made from aluminum. The factory just did it the cheapest way possible back in the 60' and 70's. FFI doesn't have enough cash to re-tool the entire case so they use factory cores. It would cost $$$ to tool up an aluminum PS box.

I bet a person could build a billet PS box. Probably cost a few grand though. It cost me $3000 to build the first SB billet timing chain cover.