Why do lecturers not just assume all students have read the book? One thing I don't understand about lectures is why the lecturer covers the material that is already presented in the book and assigned as reading material to the students. 

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?

I understand that some students may not understand the book, and a spoken lecture makes it possible to answer questions and provide clarifications on unclear statements from the book, or just to add more detail/intuition to a certain topic.

But, truth be told, those parts of a lecture make up a small, small minority. Most of the lecture is indeed spent on just repeating what the book said - except of course with much less detail, since you can't cram 30 pages of written words in a 1 hour lecture.

So would it not be 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 15 minutes, certain added remarks can be made if the lecturer feels them necessary.
Spend the next 10 minutes answering questions by confused students.
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.


This way you are both more efficient since you are not wasting time repeating stuff, and you also take care of students' questions and get some time to add clarifying remarks. You get the best of both worlds.
The answer may depend on what type of course we are talking about, here I'll focus on STEM.

The most obvious answer would be "because it would be a false assumption". Many students don't read the books. But then you get to the question: why don't the students read the books?

I'm an engineering student, and here's my perspective: in STEM courses it's quite often the case that the lectures are quite simply a boiled-down version of the books; they present the same theorems as the books, in the same order, often using the same examples, but without all the "fluff". Students don't read the books because we go to the lectures, take notes, and get all the necessary material in a document we wrote ourselves. Writing it ourselves helps us learn the material, and since it's a boiled-down version it's much less material to read through when preparing for exams.

As for professors, if they assume that the books have been read and understood, they wouldn't have much to talk about since what they planned to present was the book's material. They could use lectures for Q&amp;A sessions and practice problems but attendance would be quite low in that case and students would complain about paying for simply being told what book to read instead of being presented the material. Besides, (at least in the case of my university) Q&amp;As and practice problem sessions are typically done by the TAs on a regular basis anyway.

I imagine this may not hold for other types of classes such as history or literature in which documents are assigned to be read and then discussed and analyzed in class, but I have little to no experiences with those so I can't give an informed answer.

P.S: "Imagine that reading the book is a lecture itself". Fun fact: the word lecture is french for reading.