Why don’t they just shorten the duration of school? Why does a senior in high school need 7 hours of classes 5 days a week, but one summer later only need 4-5 h

Why don’t they just shorten the duration of school? Why does a senior in high school need 7 hours of classes 5 days a week, but one summer later only need 4-5 hours of classes 3 times a week as a freshman in college?