Disclaimer: I'm not an experienced teacher, and I certainly don't represent all lecturers, so this is mostly just guesswork based on my experience in academia.
Edit: Just to clarify, I'm not claiming that what lecturers do now is optimal, I'm just explaining why they are doing it now. However, planning and executing a good and pedagogical course from scratch requires a lot of time and effort that might not be available to many university teachers, even if they were motivated. So the current method may indeed be a "local" optimum in many cases.
Lecturer's job is to help the students learn as well as he can in the time given to him. In an ideal world, the students would do everything as the lecturer has planned, but experience has told us that this is not what happens. So the lecturer needs to adapt.
I've been TAing a course where the lectures start by a small quiz about the contents of that lecture. The quizzes aren't graded, but you get some extra points by attending a lecture (on time), so the students have some motivation to do it. And the results vary from end to end. A few students seem to know everything the course wants to cover, and a few students seem like they haven't even heard about the things in the course, and the rest are anything in between.
Why? Is the assumption that some students are actually incapable of reading? If not, what then is the point of covering the same stuff in the lecture? Imagine that reading the book is a lecture itself: what then is the point of giving two identical lectures?
The assumption is not that they are incapable, the assumption is that they are not willing to learn everything just by reading a book, for good or bad reasons. Lecturers often have chosen to target their lectures to people who, for one reason or another, prefer not to learn stuff only by reading a book. Beause students who learn well enough by reading the book don't really need lectures as much as the ones who don't.
Even the worst lecturers I have seen don't just recite the book. They allow questions and try to explain things in their own way. Another point of view often helps you if you didn't understand what the book said.
And if the lecturer's quality of teaching is so low that they really just recite the book without adding anything, I don't think the problem is the structure of the lecture, it's their motivation, skills, or time constraints.
So would it not me much more efficient to do it differently? Here's one example for a 1 hour lecture, but you can think of your own:
- Spend the first 15 minutes on recitation of the book material. It gets everyone on board, sets the tone, refreshes everyone's mind. During these 10 minutes, certain added remarks can be made if the lecturer feels them necessary.
Earlier you said you can't even cover everything in the book during one hour. So everyone who has not read the book or has not understood something will definitely not understand anything here.
- Spend the next 10 minutes answering questions by confused students.
From my experience, at this point the questions would be anything between "no questions" or "I didn't understand anything" (which looks like "no questions"). If someone has a real question about one part, I don't see how this is any better for them than asking it while the lecturer is teaching that part in the 1 hour lecture.
- You now have 35 minutes left to cover extra material not presented in the book. This could be in-depth examples or intuition of the topic just covered, advanced extensions, or just some extra topic that you wouldn't have time to otherwise cover in your course.
Books are designed to contain material that is suitable for one course, and I'd assume lecturers want to teach the material they currently have in the course. So they would probably want to spend this 35 minutes going over the material they explained in the first 10 minutes, but in more detail. So this would be like the usual book lecture you described, just structured in a different way.
This way, you are both more efficient since you are not wasting time repeating stuff,
But repeating stuff is pretty much required for learning. Yes, you get repetition when you do exercises, but reading a book and listening to a lecture about the same things is another way of learning.
but you also take care of student questions and get some time to add clarifying remarks.
Are you saying that your lecturers don't allow student questions during the lecture? I have seen lecturers ranging from horrible to excellent, but I've never met one who does not enjoy when a student asks a question during the lecture.