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 pair of scissors. Is this an English grammar rule when 2 parts are connected to function as one it’s still a pair.?
I’d say it’s less of a grammar rule and more about how things made up of two similar parts can be conceptualised
It’s called a plurale tantum or “plural-only noun”.
English grammar and spelling rules were made up on the fly by Dutch workers with a tenable grasp on the language themselves.
They just operated the first English printing presses and the owner valued quantity over quality. So they just did fucking whatever.
Source?
It was William Caxton about 600 years ago who owned it.
Everything I found just now talks about how great a historical figure he was, and implies he somehow was personally doing the work.
But if you dig deeper you should be able to find reference to the Dutch workers he brought with the press who knew how to use it and actually set the type on the presses. They were the ones actually making those books that standardized English grammar and spelling.
I don’t have the time at the moment, but if you’re interested then that’s enough to start researching