The idea is not "thrown around". It is called the Carnegie Unit: A course should be designed so that for each unit there should be 1 contact hour and 2 hours of outside-class work. This is the standard way courses are implemented in North American universities.

In my college (back in the day) they said an A student would spend 4 hours for every 1 hour. For a 15 credit student, that meant 60 hr work weeks every week and for a 20 credit student, 80 hours a week every week. It was brutal and exploitative imo. The one semester I took 12 credits, my quality of life, ability to retain material, and grades went up.

Let's face it, the average student won't do anything on their own till the night before the exam, which is a recipe for failure in any class that can't be memorized in an evening or two. If school was tailored towards the most elite self-motivated students, your analogy would apply. But something like half of gym members don't even go.

Why do you think professors give homework? It would be so much easier to just point students at study materials and leave the responsibility to them. But it's frustrating for everyone when students fail.

How is that a stockholm syndrome? Classroom hours are set low enough specifically because of the expectation of out-of-classroom work/study. The courses could meet for longer and expect all work to be done in class, but that is incredibly restrictive for planning schedules.