Why is underwear called "a pair" when it's just one item?

Phase 1: Collect underpants
Phase 2: ?
Phase 3: Profit

vs

Phase 1: Collect pairs of underpants
Phase 2: ??
Phase 3: Double the profit!

Like, do I really have to explain this to you guys?

Well, if they held 2 testicles AND 2 butt cheeks they’d be a quad of underwear.

A pair of briefs would be enough for two people. Because briefs is singular…

Zombies do not care about pants, singular or plural.

As far as pairs of pants go, so the Zombie may return to sleep…

…do not pants derive from leggings, which were individual and so in pairs.

So someone devises to stitch them together to the loin cloth now the pair is joined together?