You're conflating education work with productive work. School is about learning, for one's personal fulfilment. You're supposed to do that mostly on your own time. School is just a way to keep you motivated to learn a particular thing, and the notion of homework is necessary to reinforce that learning happens on your own and in a classroom.

But actually mostly on your own. 45 minutes twice a week in a classroom is just not enough time to learn anything of substance and depth. The Thursday-to-Tuesday memory loss is especially profound; Tuesday class is mostly spent reminding everyone who didn't do their homework what we had talked about last Thursday.