why certain nouns are called collective nouns?

Answer 1

Nouns themselves are not inherently 'collective nouns', they become collective nouns by their use; they give a clearer picture of a group of people or things or they can add color to a description. Some nouns do lend themselves for use as collective nouns, but those nouns can be used in other contexts as ordinary, common nouns. Examples:

Used as a collective noun: There is a crowd of protestersin front of the building.

Used as an ordinary noun: The protesters in front of the building are drawing a crowd.

Used as a collective noun: A small school of fishgathered around my ankles.

Used as an ordinary noun: I attend a very small school.

Used as a collective noun: There is a bench of judges who determine the winning entry.

Used as an ordinary noun: I was told to sit on the benchoutside the principal's office and wait to be called.