That's not usually the way textbooks work --- they often try to support many instructors who each want to cover more or less the same core plus some nonoverlapping material that the instructor wants to teach. When you add up all of the nonoverlapping material, it becomes less clear what's "most worth your time." Some textbooks present the material as "here's the core" and "here's some other stuff" but I've mostly seen that for advanced texts -- books for high-level undergrad & grad classes.

Compare something like Schaum's Outlines to an undergraduate text for the same material for an example of what I mean. The outlines are pretty pruned down.

edit: just to add one more thought: we read all the time about "opinionated software." Pruning down the material requires understanding and "opinion" in the same way --- textbook authors almost invariably have the understanding, but are usually economically discouraged from imposing their opinion on the material because it can discourage book sales because instructors with different opinions tend to choose a different book. Lecturers don't have the same concerns, so they can prune more aggressively. Authors of textbooks that will never sell well (e.g. the advanced texts I mentioned earlier) don't have the same concerns either since they're never going to make a substantial amount of money off the book, so they can prune pretty heavily too.

Most of them do not. My impression is that they try to cover much more then usual semester long course.

Plus, while it may make sense for textbook to contain five different explanations, theories or proofs or whatever on X, it makes even more sense for professor to require only the most important of them.