Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, or *to what extent* something happens.
* "Rarely" tells us how often something happens (infrequently).
For example:
* He rarely visits his family. (modifies the verb "visits")
* The food was rarely cooked properly. (modifies the adjective "cooked")
* She rarely speaks about her past. (modifies the verb "speaks")