Why would brassiere be anything but singular?
Panties is the diminutive feminine form of pants which is short for pantaloons, which refers to the skinny legs and tight clothing of the Italian theatrical character type known as a Pantaloon.
You know, I did go to university and I have a degree in linguistics. I don't need Google for every little thing.
Good question...prolly the same reason our noses run and our feet smell!...lol...
Nobody seems to have anything! I don't really know either, and besides I'm a guy, but but I speculate that it's because panties (or pants) seem more like a pair of something because you have to slip one leg into them, then the other (unless you're good at doing it simultaneously) whereas a bra would be put on in just one motion (except maybe if you're Dolly Parton or something) ;-)
I don't know but thats a good question. :)
Geeee, I haven't come across this question before. Can't wait to see what answers people will have for you.