Here's why:
* Common nouns refer to general people, places, or things. They are not specific.
* Collective nouns refer to a group of people or things considered as one unit.
"Grammar" is a general term for the rules of language. It's not a specific group of things.
Examples:
* Common noun: The dog barked. (refers to a general dog)
* Collective noun: The team won the game. (refers to a group of people as one unit)
Let me know if you have any other grammar questions!