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?
That’s interesting. The Encyclopaedia Britannica appears to contradict this, stating:
Source: see the reply by @Nougat@fedia.io to you.
It is also true that pants were originally two separate legs, lashed as shown. But that might not be the reason we refer to them as a “pair of pants.”