Why do you have spend your precious time off to be able to take courses that you want? Why can't you just take the courses you want while staying on schedule?

Why do you have spend your precious time off to be able to take courses that you want? Why can't you just take the courses you want while staying on schedule? IMO dipping into summer holidays more or less equals unpaid overtime at work. Society can force one to do that, but it's unfair, draining and not that much productive.