Why can't boys keep cool in the summer by wearing short skirts like girls can?

Because It's 'Socially-Unacceptable' for a male to wear female clothing, whereas it's perfectly normal for females to wear male clothing.

However, you can wear a Scottish kilt and not have to wear female clothing.

But:

Scottish kilts are big, heavy things, though. In the summer I just long to wear the sort of light floaty short skirts a girl can. Even in shorts and underwear a boy's got the top part of his legs covered, and two layers covering the worst place where a boy can get hot. Girls can wear a short skirt and no underwear which is really no layers at all. It's not fair that we can't do the same and have to be hot and uncomfortable all summer.