Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* "Somewhat" indicates a degree or extent. It tells us that something is true to a limited or moderate degree.
Example:
* "The weather is somewhat cold today." (modifies the adjective "cold")
* "He somewhat agreed with her proposal." (modifies the verb "agreed")