Here's why:
* Subject: The subject is the person, place, thing, or idea that the sentence is about. It's who or what is performing the action or being described.
* Predicate: The predicate tells us what the subject is doing or being. It contains the verb and any other information about the action or state of being.
Example:
* Subject: The cat
* Predicate: is sleeping on the couch.
Together, they create the complete sentence: "The cat is sleeping on the couch."
In summary, a predicate alone is not a complete sentence. It needs a subject to tell us who or what is performing the action or being described.