Pants, jeans, trousers, britches, Levis, dungarees -- they're all plural. But why. It's not like anyone ever has one pant, jean, trouser, britch, Levi, or dungaree. And I understand that they have two legs, but shirts have two sleeves, and blouse, top, shirt, chemise, and sweater are all singular. As is jacket, coat, windbreaker, yadda yadda ya.
Even underwear that has no legs (just leg openings) get pluralized: boxers, jockeys, shorts, panties. How is a person supposed to put on a pair of panties when there is only one of them? What are the two halves of the pair here?