Why don't you then start listing steel cases which use thicker than 1mm sheet?
I haven't yet seen any advertisings, specs or anything about steel case which would use thicker material no matter how many times I've asked.
While steel weights thrice the alu it's still not good in stopping vibration because thin metal sheet is thin metal sheet regardless of what it's made of and as non-elastic material steel conducts vibration quite well.
Giving real vibration stopping ability with layer of vibration absorpting material or through extra rigidity makes it heavy with all capitals. (Antec P18x/19x already use alu-plastic-alu layer structure to keep weight of side panels reasonable)
If weight is an issue and budget isn't so limiting alu+bitumen gives lot better vibration resistance per weight.