The word panties makes my skin crawl. It's un-necessarily infantilizing an undergarment. I don't see the problem with just saying underwear. Most words that end in 'ties' is an attempt to cute up something or diminish. To infantilize and sexualize a piece of clothing all in one word, creeps me out.

Why do you drive in a parkway and park in a driveway?

because bra is short for brassiere, and it's one unit, named not after boobs but after arms, which admittedly also come in pairs -- but it doesn't go down the arms. panties is a diminutive of pants, which traditionally DO go down the legs too (think bloomers) and so it's plural because of the legs. does it make sense? no! but you didn't say what sensible reason was there, only why! that is probably the real reason, whether or not it makes sense.

g

Don't worry about it