So then we get down to what is a pair of pants? In America its also trousers and in British terms its under-pants. When we ask for a pair of pants are we really asking for trousers and under-pants? Since Pair means two or a matched unit like gloves or socks.
What happens with pants and it having to do with breathing? How did trousers become pants? According to Wiki Pants, the plural ancestor of the "Pant" were invented in 1904 by William P. Pants , but this wiki page seems suspect to not being credible since doing a Google search only returns that page for William.
So it still comes down to why do we call trousers, pants?