Three more reasons:
* It takes time to figure out which details are essential and which are not. When extemporizing about a topic, which happens frequently no matter how well you prepare, you may throw in extra material because you don't have time to figure out exactly which bits are necessary to form a coherent picture for your listeners. Academics especially will err on the side of verbosity: adding a few extra facts feels like a lesser evil than leaving an important point unaddressed.
* When writing a book, you have time to pick the very best and richest examples, accomplishing as much as possible with each detail. In a lecture you may end up using many details, because each detail only illustrates a small part of what you're trying to say.
* Lectures are delivered live, with no opportunity for rereading, so repetition is important. In a book, a difficult point should be stated as clearly and precisely as possible with the expectation that a reader who is confused will reread the point many times until it becomes clear. In a lecture, you can't simply repeat yourself five times because a few of your listeners didn't understand, but you can't abandon those listeners entirely. You have to give them time to grasp the concept while at the same time keeping your other listeners engaged. Going over many examples and explaining the connections to related ideas serves the dual purpose of clarifying your point for those who are floundering and providing extra information for those who understood you the first time.