Singulars, plurals, Jockeys and panties

I'm not a grammar fanatic but, as a writer, I pay attention to grammar and even to some of
the more arcane rules and oddities of English grammar.
Perhaps it's because I have a week off or because the never-ending rain here is triggering
my seasonal affective disorder, but I got to thinking about plurals and underwear.
That's right. Plurals and underwear.
Socks are plural because we buy them in pairs. Yet if one sock has a hole in it, we don't say "My
socks have a hole," but "this sock has a hole."
But underwear is a single item, so while is it that we call what we wear Jockey shorts or panties
rather than a Jockey short or a panty?
Some would say, "Well, that's because we refer to a pair of shorts or pair of panties and "pair"
is just understood when we say "Jockey shorts" or "boxer shorts" or "panties."  
But why is it a "pair?"  
Well, you reply, probably because there are two leg openings.
Well, I rejoin, then why don't we say a pair of shirts because a shirt had two arm openings?
The same question could be asked about slacks or walk shorts?
Or jeans.
Would someone say, "I'm going to wear jeans and shirts."?
No, they would say, jeans and a shirt.
Why not a jean and a shirt?
Or a jean and a short?
Why say a bra and panties?
Why not a bra and a panty?
Why say a t-shirt and Jockey shorts?
Why not a t-shirt and a Jockey short?

And wear either combination under a shirt and a jean?

With a pair of socks, of course.

It's enough to make me give up wearing a jockey short and wear a panty.
Actually that kind of makes sense. 
Time to stop writing.