I was wondering the other day why words that describe things that go over the lower legs tend to be talked about as a pair of
things, even though they are only one cloth, while the things going over the upper body tend to be singular.

It goes beyond that, Nancy. Why, in English, is "hair" both singular and plural, whereas in some languages one uses a plural form when referring to more than one hair? Yet, when speaking of bilaterally symmetrical body parts above the waist, we always use a plural form, e.g. eyes, nostrils, and ears. 'Tis strange.