Why is one garment called a pair? Even the earliest pair of pantaloons were attached in the middle. Linguists call this a plurale tantum, Latin for plural only. English has lots of pluralia tantum: electronics, odds, surroundings and thanks. Items that have two symmetrical or attached halves are popular examples, like scissors, pliers, and glasses. They all are a pair despite being a single item.
From:: Pairs of pants.