Even if you only take courses that aren't wastes of time, there just isn't that much to do on a typical day. I remember when I was in school I was taking something like 5 AP courses senior year but still had multiple study halls per day, a useless homeroom which was basically a buffer for people getting to school late, a gym class, and a 40 minute lunch. Almost half the day was time not spent in lecture. A lot of the time is literally babysitting.

This is how people can go to college and spend comparatively little time in class yet learn more than they did in high school.