Here's why:
* Nouns are words that name people, places, things, or ideas.
* Pronouns are words that stand in for nouns.
Examples:
* The cat sat on the mat. (The cat is the subject, a noun)
* She is reading a book. (She is the subject, a pronoun)
It's important to note that a phrase or clause can also act as the subject of a sentence, but these often contain nouns or pronouns within them.