Why is 'bra' singular and 'panties' plural?
A bra is a one piece garment. In the old days, any kind of pant was two pieces that the person (usually a man) put on separately. The tradition of keeping pants or panties plural stayed.
Because bra's have to hold two breasts…wait, never mind. Because english is a weird language!
Hmm that doesn't make much sense! But it doesn't affect me either way.
Your question doesn't really make sense. Why is it like that? Because that's how people made it....
Because congress made it that way.