Here's why:
* Objects are the nouns or pronouns that directly receive the action of a verb. For example, in the sentence "The dog chased the ball", "ball" is the direct object.
* Objects of prepositions are nouns or pronouns that follow a preposition and complete its meaning. They answer the questions "who?" "what?" "where?" "when?" or "why?" For example, in the sentence "The dog ran under the table", "table" is the object of the preposition "under".
Prepositional phrases (preposition + object) act as adjectives or adverbs, modifying other words in the sentence. They don't receive the action of the verb.
In short, objects of prepositions are not objects in the grammatical sense, but rather elements within prepositional phrases that modify other words.