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?
A jumper is a bad example, because it’s not just made up of sleeves.
A bra is a much better example. But I was never making an argument that all things made up of two parts are referred to as a pair, just my thoughts on the rationale behind cases where we do.
Understood.