Here's why:
* Noun: A noun is a word that names a person, place, thing, or idea. "Names" refers to a collection of names, which are things.
* Adverb: An adverb modifies a verb, adjective, or another adverb. It tells us how, when, where, or to what extent something is done. "Names" doesn't describe how, when, where, or to what extent something is done.
Example:
* Noun: "The list of names was long." ("Names" is the subject of the sentence, naming a thing.)
* Adverb: "The runner finished the race quickly." ("Quickly" tells us how the runner finished.)