A typical course might have three hours of lecture per week for ten weeks. That’d be 30 hours, except usually around 6 sessions are skipped for exams, reviews, holidays, etc. Call it 24 hours. It’s just not enough time, not enough bandwidth to communicate all the details in a serious block of material.
Speaking quite rapidly, I can do perhaps 100 WPM. But lectures have lots of pauses, and questions, and starting/stopping time. The lecturer usually isn’t speaking flat-out the whole time. An average of 30 WPM is probably more realistic across the total time. So that’s 43k words in 24 hours. Even that’s probably an over-estimate. But it’s also just not enough. A typical full-text page in a textbook has about 500 words on it; 43k words would be just 86 pages. Cognition - Reisberg, the textbook for UCSD’s COGS 101b, has 586 pages in the “body” of the book, at least half of which are “full-text.” The rest have imagery or special sections or are blank, but of course, there are still probably 50k+ words on the less dense half. Probably at least 200k words in the body of the book.
Part of the trouble here is that there simply aren’t enough lecture-hours in the course. You could solve this by just adding a lot more hours—in this case, maybe quintupling the lecture-hour count to around 120 hours. But this wouldn’t be a good idea: you could do a solid pass or two on the full textbook yourself in substantially fewer hours, and you’d probably end up with a much better grasp of the material.
Lecturers usually respond to this problem by covering the material quite shallowly. But to a novice in the topic, it’s often not clear that this is what’s happening. So a common experience of attending a lecture-based technical course is that you feel you’ve learned a great deal, but when you look at your notes or try to explain a specific topic in detail, you find that you don’t really understand.
I recently viewed the lectures for COGS 101b (see: Log: improve background domain knowledge) and took extensive notes. It felt like I’d learned a lot… but I thought I’d work through the textbook, too, just to cover my bases. I discovered that I really didn’t understand almost any of what was being discussed in any detail. I was more or less just learning the bullet-point-ish “take-aways” of a lot of big ideas. I certainly wasn’t putting myself in a position to generate new ideas based on any of this material. In hindsight, I wish I’d just started with the textbook.
Of course, lectures are extremely useful for reasons other than information transmission: e.g. Lectures can offer a glimpse of what it’s like to think like an expert.