Here's why:
* Nouns are words that name people, places, things, or ideas. They are the core of what the sentence is about.
* Pronouns stand in for nouns, making it easier to avoid repetition.
Example:
* The cat (noun) sat on the mat.
* He (pronoun) ate the fish.
While nouns and pronouns are the most common, there are some exceptions where a phrase or clause can function as the subject:
* Running a marathon (phrase) is a difficult task.
* What you said (clause) surprised me.