Why do lecturers not just assume all students have read the book?
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.