By and large undergraduates at university just refuse to put in the hours for study. If you assign any reading of more than 50 pages, maybe 5% of the class will read it. Some of them are incapable of focusing for that long, some of them simply refuse to. what they do instead differs from person to person, but what is clear is that students simply expect less work and given the rampant grade inflation professors often go along with the students expectations.
Even if a student takes a more moderate approach, workloads can be highly unpredictable (even to the professor), which creates the same problem.
I don't know where some of the other commenters get the idea that students these days don't take the time to study. I can't comment a lot about Harvard, but at my school, it was difficult to come across a student who wasn't taking a full load of courses (more often than not double or triple majors), involved in several extracurricals and often doing work study.
A lot of this was poor time management and over-stress on my part, but that's what I was in college to learn to manage. If I went back Quantum-Leap style I would be able to chew through everything a lot easier.