Here's why:
* Collective nouns refer to a group of people or things considered as a single unit (e.g., team, family, flock).
* "Word police" is a metaphorical term used to refer to people who are overly concerned with language correctness or who try to enforce strict linguistic rules. It's not a specific group with a defined membership like a team or family.
While "word police" might evoke a sense of a group, it's more of a general, figurative term than a true collective noun.