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?
For some items like glasses it’s very clear why they are pairs; if you can have a reading glass (which is an antiquated way to refer to a handheld magnifying lens, for example) then you can certainly have a pair of reading glasses because it’s the two pieces of glass which are plural.
For trousers there are no certain answers, but I’d suggest it’s very much with with how we conceptualise their function. For 90% of their height trousers are split and cover the legs, of which we have two, only joining right at the top.
For shirts you might think it’s the same (two arms right?) but it’s a completely different story because the primary function of a shirt isn’t to cover the arms but to cover the torso. So it’s singular. And gloves of course are distinct, so it’s back to pairs.
What about a brassiere?
Same logic, the bra itself as a whole is a chest covering, so like a shirt it’s not a “pair”
Not that there always has to be logic in these things, etymology sometimes defies that.
A bra does have a “pair of cups”, though!
A pair of pliers is just one tool.
And an au pair is just one person, usually a woman, and they aren’t made of a piece of gold, much less two.
Checkmate, linguists!