Here's why:
* It names a thing: A subcommittee is a smaller group within a larger committee.
* It can be the subject of a sentence: "The subcommittee met to discuss the budget."
* It can be the object of a verb or preposition: "We formed a subcommittee to study the issue."
While "subcommittee" is formed from the prefix "sub-" and the noun "committee," it functions as a single noun in its own right.