Same goes for glasses and scissors. Items that have two distinct symmetrical parts are often referred to as a "pair" even if it is technically one item.
Why do we call it a pair of underwear and a pair of pants, when it is one garment?
in the early 19th century women's underwear consisted to two separate legs joined at the waist. They really were a 'pair'.
http://www.localhistories.org/underwear.html
not sure how true it is but that's what I found lol