Here's why:
* Pronouns are words that replace nouns. They don't name specific people, places, or things like nouns do.
* Common nouns are general names for people, places, or things (e.g., dog, city, book).
* Proper nouns are specific names for people, places, or things (e.g., Fido, London, Harry Potter).
Examples:
* Pronoun: "He" replaces the noun "John."
* Common noun: "Dog" refers to any dog.
* Proper noun: "Fido" refers to a specific dog.
Therefore, pronouns are a distinct category of words that function differently from nouns.