get used to it

What we call a pair of pants is in fact one object so logically it makes very little sense to refer to them as a pair. I suppose they may have once been referred to as "a pair of leg sheathes joined at the middle by a crotch bit," but perhaps that became a bit cumbersome, and since "pant" was already in use to describe that activity engaged in by dogs and some deranged individuals, eventually "pants" was settled upon as a reasonable compromise. Then again maybe not. Most English speaking have no idea of the answer to that question because they don't need an answer to it. The best answer to a question like "Why do we call them 'a pair of pants?'" is actually "I dunno. English is crazy. Get used to it." While you may or may not find my reasoning humorous, I am not, in fact, kidding. I teach adults and that is what I would say. There are much more useful things to focus on and they need to understand that.