Vice versa, they can't wear skirts for hot days, but gals can.
The answer is imho, holdovers from more sexist days, and a general attitude of not wanting to be seen as effeminate/photographed by creeps.
Even though guys have been wearing kilts for many years, in various cultures; and there's AI slop now anyway that keeps 'em busy instead.
Iirc it's also allowed in some countries. Naturism is bigger in them.