Here's why:
* Collective nouns refer to a group of people or things considered as a single unit (e.g., team, family, committee).
* Pronouns replace nouns to avoid repetition (e.g., he, she, it, we, they, us).
"US" represents a group of people, but it's not the noun itself. It's a pronoun replacing the noun phrase "we" or "the people in this group".
For example, consider the sentence: "The team won the game."
* "Team" is a collective noun, referring to a group of players.
* "They" is a pronoun replacing the noun "team."
Similarly, in the sentence "The US is a powerful country," "US" acts as a pronoun replacing the noun "the United States."