Here's why:
* Collective nouns refer to a group of things considered as a single unit, like "team," "flock," or "herd."
* Rocking chair refers to a single piece of furniture.
If you were talking about a group of rocking chairs, you might use a phrase like "a set of rocking chairs" or "a collection of rocking chairs." These phrases would use collective nouns like "set" or "collection" to describe the group.