Those same questions always occur to me. Plus, why would Jesus have spent so much time teaching if He already knew what everyone was going to get on the exam, if you will, and there wasn't any way for any of them to change that.
Attempting to teach indicates the belief that someone can learn and thereby modify some aspect of their behavior. To what end becomes the obvious question.
Oh for heaven's sake....again, because God finds the elect through the truth being preached. How easy is that to understand...it is VERY easy!