Why do think Ishmael doesn't simply lecture and tell his "student" all of his ideas up front?

A teacher's job is to teach..... to encourage students to think for themselves and come to their own conclusions..... form their own opinions, use what they know to formulate new ideas. Ishmael presents the narrator with the knowledge and material he needs to formulate his own take on the things he's been taught to believe. It's one thing to teach a subject.... it's something completely different to apply it. Ishmael gives the narrator the framework.... it's up to the narrator to "live" the lesson, thus, passing it on.