We say a pair of jeans, a pair of pants, a pair of scissors, etc.
Why don't we say a pair of bras even though the bra also consists of two equal and symmetrical parts?
That is indeed interesting. We do, however, say "a pair of tits," "gazongas," or similar. "Pair" is not necessary, but it's sometimes added.
We don't say "a pair of shirts" but we say a man is "in his shirtsleeves." (Or once did; I doubt anybody knows what that means these days.)
I seem to remember reading an asterisked explanation of the phrase in some book we had to read in high school. Of course, at that age many phrases require an asterisk.
But honestly, I don't think I've ever heard anybody say it over here. Can I get a witness?
Over her we do use 'in shirtsleeves'. A phrase that is definitely on the wane is 'shirtsleeve order' - used chiefly by people with a military background (and by people who went to fee-paying schools that had a "Corps" - boys playing at being soldiers, but with real - if old - equipment).
When it was hot a soldier could not remove his uniform jacket unless his superior had announced 'shirtsleeve order'. The Test Match Special (BBC, cricket) commentators have the appropriate background, and they frequently say things like 'It was shirtsleeve order at Lords today'.
So, that means it was really hot?
I love to listen to the American football commentators, esp. the "color guys," try to find something clever and informative to say. Best Ever: "The quarterback's down in the end zone nursing a hurt calf."