People do ask the most intriguing questions.
I've looked at the entry in the Oxford English Dictionary , which suggests that the form pair of pants was standard right from its earliest use. Commentators referred to them when they first appeared as being a combination of breeches and stockings. Later the word was applied to fashionable tight-fitting trousers. According to several costume historians who have helped me with this reply, the answer to all this conventional plurality is very simple. Before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg.