Why do we say a pair of jeans to mean one pant?

A pair of jeans always means a pair of pants, never one pant.

The pant is basically the leg covering, so two legs equals pants. The rest is there to hold the pants up.

It probably evolved from chaps that were individual pant legs, or something like that.

Oh and a good comparison is a pair of glasses, referring to the glass instead of the whole item.

Apparently, centuries ago, pantaloons (pants) were separate articles, one for each leg, joined together at the waist by a belt. Eventually they became one piece of clothing but the "pair of" stuck.

Other examples of pairs: scissors, tweezers, shears, pliers, eyeglasses, binoculars (developed from putting two telescopes together - and since "bi" already denotes two, really makes no sense), shorts, panties, handcuffs, headphones.

I have no idea what you talking about...truly...

I wouldn't pretend to try to answer the question, except to say it's because English is nuanced. It's also extremely hard to learn for non-speakers for this very reason, by the way. I mean, "Feb-ru-ary," what in the holy hell is that?!! I highly recommend reading "The Mother Tongue" by Bill Bryson if some of the backwards nuances and idiosyncrasies of the English language are of any interest to you whatsoever.