With a sentence like, "He sat on the couch," is sat still being used as an intransitive verb since, in general, prepositional phrases cannot act as direct objects? To me, the prepositional phase "on the couch" seems to be behaving like a direct object. If it is not behaving like a direct object, then I should not be able to construct a passive voice version of the sentence (i.e., "The couch was sat on by him"). To be sure, no one would actually use this passive voice construction, but it was nonetheless possible to create.
What am I misunderstanding here?