Here's why:
* Collective nouns refer to a group of individuals considered as a single unit. Examples include "team", "flock", "army", "family".
* Wrist watches are individual items. You don't think of them as a single entity, even if you have multiple watches.
Instead, "wrist watches" is a plural noun. It refers to more than one watch worn on the wrist.
Let me know if you have other words you'd like to check!