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.

I’d say it’s less of a grammar rule and more about how things made up of two similar parts can be conceptualised

They’re made up of two similar parts which form a pair, unlike a jumper