Here's why:
* Indefinite pronouns refer to non-specific people, places, or things. They don't have a specific antecedent (the noun they refer to).
* "Many" and "few" indicate quantity, but they don't tell us exactly how many.
Let's see them in action:
* Many people came to the party. (We don't know how many, just that it was a lot)
* Few students finished their homework. (We don't know how many, just that it was not many)