I was wondering the other day why words that describe things that go over the lower legs tend to be talked about as a pair of things, even though they are only one cloth, while the things going over the upper body tend to be singular.

I can understand that pantihose should be a pair, because they started out life as a pair of stockings. But why should shorts, underpants, pants, trousers, coulottes, etc be talked about as a PAIR, while a braissere, a blouse, a shirt, a shawl, etc, are always singular?

Any thoughts?