In American English, shorts and jeans are always referred to in the plural even though the thing they refer to is a single object. No one says short or jean for one of shorts or jeans. The word jean can be used as an adjective to refer to a jean jacket made from denim like jeans, but that’s a different use. Sneakers are usually referred to in the plural unless you’re specifically referring to just one of them for some reason like I lost a sneaker. The word shoes is more common now than sneakers but the plural singular use is the same. Can you think of other examples of singular use of things that are actually plural? And what did you mean by hill?

I agree with Tim's answer. You shouldn't say "a sneaker" unless you are talking specifically about one shoe, and you shouldn't say "jean" unless you're using it as an adjective to describe a type of material. "Shorts" is always plural, never singular. You can't say "a short." It doesn't mean anything, unless "short" is an adjective describing something else ("a short story," "a short vacation," "a short jacket"). Like Tim, I don' t know what you mean by "hill."

Most items that have identical sides, parts or sections are described in the plural form. In English, we have pairs of trousers, shorts, slacks, pants, tights (leotards), nylons, leggings and of course, shoes, such as high heels, runners, sandals, loafers and flip-flops.