Here's why:
* Pronouns replace nouns. They stand in for nouns that have already been mentioned or are clear from context.
* Common nouns are general names for people, places, or things. They are not specific individuals or objects.
Example:
* Common noun: "dog"
* Pronoun: "it"
You can't simply say "it" instead of "dog" without any context. You need to have already established what "it" refers to.
However, you can use a pronoun to refer to a noun phrase that includes a common noun:
* Noun phrase: "the big brown dog"
* Pronoun: "it"
In this case, "it" is replacing the entire phrase "the big brown dog."
In summary: Common nouns and pronouns have distinct roles in language. While you can't turn a common noun directly into a pronoun, you can use pronouns to refer to noun phrases that contain common nouns.