The explanation is often a simple one: there is a big difference between "listening" and "doing".

It's very easy for a student to sit in class, passively listen to their teacher, and convince themselves that they understand the content. However, in actual fact, most students have only a very superficial understanding when they leave the classroom, not even close to the level they require to succeed on exams. They are still very distanced from the content, haven't really interacted with it themselves, and have no idea about the variety of questions that can be asked of them come exam time.

Genuine, deep learning comes from sitting down and actually doing (and then marking) practice questions. This way, they are actively engaging with the material and exposing themselves to the different ways in which it can be tested. Furthermore, they have an objective measure of their understanding: whether they are getting the correct answer.

If exams involved simply sitting in a classroom, gazing at a whiteboard, and nodding one's head, then I'd see no problem with students thinking that listening to their teacher in class is all they need to achieve a good grade. Of course, the reality is that exams are nothing like this. Since we all know that exams involve students working independently to solve unseen problems, surely the most effective method of exam preparation is...working independently to solve unseen problems?

I've always thought that the sporting arena offers a great analogy to this point. Roger Federer, for example, never won a major solely by sitting and listening to his coach. He actually picked up his racket, walked onto the practice court, and hit thousands of balls every single day. It doesn't matter how many times he was told to toss the ball at a certain angle, hold his racket in a particular position, or keep his head still through the shot - if he didn't actually practice doing those things, he never would have been able to replicate them in a match.

Relying on teacher explanations without independent practice also leads to a lack of personal responsibility. If students aren't a fan of their teacher, they tend to blame their poor understanding on him/her. By contrast, the top students in the state take accountability for their own understanding by going home, solving problems, and learning the content by applying it themselves.

With all of these negative consequences, it seems relevant to question why students so often fall into this trap in the first place. Put simply, it's a much easier route for them to take. Passively consuming content with no chance of being "wrong" requires a much lower effort level than having to actively think about unfamiliar problems which might expose a lack of understanding.

None of this is to say that listening to teachers in class is unhelpful - in fact, quite the opposite. It's a very important part of the learning process, whereby a student's mind begins to grapple with a concept for the first time. However, students relying solely on their teacher are likely to be disappointed. A big part of learning is done at home with a pen in hand, solving problems exactly as will later need to be done in exams.