In addition to what FredCapp said, direct objects receive the action of a transitive verb. The prepositional phrase "kiel vi" is acting adverbially. How do I want to act? I want to act like you. "Act/agi" is not a transitive verb. Nothing is being acted upon.
An example of a transitive verb with a direct object that is put in the accusative case would be "The boy throws the ball". Here, "the ball" receives the action of "throws". What was thrown? The ball. We can't ask "What was acted?"