The other day my son asked me why we say "a pair of pants" when pants are only one thing. Of course I immediately said it is because a pair of pants has two legs...just like a pair of glasses has two lenses and a pair of scissors has two blades. "Why not a pair of t-shirts then...t-shirts have two sleeves," he questioned.
According to Michael Quinoin at World Wide Words, pants are a pair because, "before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys’ chaps). The plural usage persisted out of habit even after the garments had become physically one piece. However, a shirt was a single piece of cloth, so it was always singular."