If pants are called a "pair of pants," why aren't bras?

In American English "a pair of pants" means trousers but in British English, pants as in underpants is singular,so bra and pants go together as they are underwear ,both singular,not a pair of underpants

Really? I'm British and I'd say "pair of [under]pants".

I am also familiar with the phrase " a pair of pants" or " a pair of underpants", just like a "pair of knickers".
If you think about it, you can't have "a pants" only a pant. The same can be said of "a knicker"
Pants is(are) plural.

Then again you also have a "pair of trousers"

I believe it stems from the times when they were 2 separate garments, 1 for each leg, and it was only later that they became joined at the crotch and became a single item.

I just checked in the dictionary and "pants", "knickers" and "trousers" are all PLURAL NOUNS.

However bra is singular, bras are several of them.

Don't forget that bra is short for brassiere, which itself is singular.