First thing I found on google, this applies to goggles as well.
When you say "glasses", it is plural. A "glass" is one of the lenses. "Glasses" would refer to both of the lenses, and a "pair of glasses" doesn't mention anything about the frame, but it mentions the two lenses. The frame is the object that joins the two lenses so you don't have to wear contacts and so it can stay on your head. When you say "a pair of glasses", you are referring to the "pair of lenses", not the frame. The frame holding the two lenses makes us think that it is one object, when the word "glasses" refers only to the lenses, not the frame. That is why it is called "a pair of glasses".