Here's why:
* Common nouns refer to general categories of people, places, or things. "Courthouse" is a general term for a building where courts of law are held.
* Collective nouns refer to a group of people or things. While a courthouse contains a group of people, "courthouse" itself doesn't describe the group.
Examples:
* Common Noun: The courthouse is located downtown.
* Collective Noun: The jury reached a verdict.