A friend of mine, Bill Biermann asked me why pants and shorts are always plural while a shirt is singular. It sent me to the web, my favorite personal library. It seems that in the beginning, pants, shorts, trousers, knickers, and the like were made of two pieces of material and fastened at the waist, like modern chaps. The plural just stuck. Shirts were always made of one piece of material. So, we have one shirt and one (pair of) pants.
Incidentally, pants is short for pantaloons, which in the beginning were closer in shape to stage tights. The name comes from a Venetian character in Italian comedy, who was the butt of the clown’s jokes and who always appeared as a foolish old man wearing pantaloons. Later the word was applied to fashionable pants.
Trousers came into the language in the seventeenth century from the Gaelic trowse, a singular word for a slightly different garment and a later version of it was trews, taken to be a plural because of the final s. Breeches, or commonly britches, has always been plural and were originally warn as undergarments, then the name was given to shorter pants that ended about the knee and were fastened at the bottom with a broach or string, Breeches is really singular word that uses a plural form.
Some tailors, such as in London’s Savile Row, often refer to a trouser and the singular pant. A few fashionable tailors in the US also use the singular. I am sure they do it, just so people keep asking the question.