Here's why:
* Adjectives describe nouns. For example: "The test was easy." ("Easy" describes the noun "test")
* Verbs show action or a state of being. For example: "I easily passed the test." ("Passed" is the verb, "easily" is an adverb modifying the verb)
While you can use "easy" in phrases like "it's easy to do," in this case, "easy" is part of a phrase acting as an adjective describing the action of "doing".