You can’t wear one leg each from two different pairs of jeans and go about your daily business, like you could for two pairs of shoes or socks, each of which is independent from the other, albeit left and right specific in various cases.
The same is true for a pair of reading glasses.
Whilst it’s obvious that both glasses and jeans (and pants in general) are referred to as being a pair, due to the two legs and eyes aspect, we don’t refer to a jumper as a pair of jumpers, unless there’s physically four sleeves attached to two bodies.
Why is that and where else does this occur?
Pants used to be two parts that were joined by lacing them together like shoes at the crotch. So weaving lashed together from the belly button to the groin to the ass and back up to the lower back.
I believe puffy white undershorts would be worn underneath, and sometimes a codpiece.
Another possibility is that “pants” comes from “pantaloons.” Like some other nouns (“scissors” comes to mind), they appear to be plural, and so people tend to attach other designations of plurality to them.
https://www.britannica.com/story/why-do-we-say-a-pair-of-pants
That’s a whole image right there … got any evidence of this?