Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They describe how, when, where, or to what extent something happens.
* Many sentences express simple actions or states of being. These sentences might only have a subject, verb, and possibly an object, with no need for an adverb to provide additional information.
Examples:
* Simple sentence without an adverb: *The cat sleeps.*
* Sentence with an adverb: *The cat sleeps peacefully.* (The adverb "peacefully" modifies the verb "sleeps")
Let me know if you have any other questions about adverbs!