Here's why:
* The predicate is the part of a sentence that tells us what the subject does or is. The verb is the heart of the predicate, expressing the action or state of being.
* Without a verb, you don't have a complete thought. For example, "The cat" isn't a complete sentence; you need a verb to tell us what the cat is doing: "The cat sleeps."
Example:
* Sentence: The dog barked loudly.
* Subject: The dog
* Predicate: barked loudly
* Verb: barked
In short, the predicate is incomplete without a verb.