Here's why:
* Indirect objects answer the question "to whom" or "for whom": They show who or what is receiving the action of the verb.
* Adjectives modify nouns: They describe the qualities or characteristics of a noun.
* Adverbs modify verbs, adjectives, or other adverbs: They describe how, when, where, or to what extent an action is performed.
Example:
* "She gave him a book."
* "Him" is the indirect object because it receives the action of giving.
* It does not modify "gave" (the verb) in the way an adverb would, nor does it describe "book" (the noun) as an adjective would.
In summary: Indirect objects are a distinct grammatical element that functions differently from adjectives and adverbs. They provide information about the recipient of the verb's action.