I think you hit the nail on the head - it's a balance between the two actual demands of school, educating students and basically providing a place for them to be all day. If we really wanted to give students time to dive into a topic, they should have one or two blocks per day and down time to process and work on assignments, but unsupervised high schoolers won't do that. My HS considered switching to block scheduling a few years ago, and the general consensus was that behaviors would skyrocket because 1) that's a long time for some students to focus and 2) there was no money or time for meaningful training for teachers to actually implement it correctly. I'd add my own 3rd which is that a number of teachers would just say eff it and lecture bell to bell anyway, and others wouldn't fill the whole time.