If all you do is absorb lessons without the underlying practice, the material will evaporate from your brain within a short time.
Imagine if you had only seen division or multiplication being done on the blackboard by the teacher, and never practiced it. Today, you'd at best have a faint, dim recollection of having seen such a thing, and you'd be powerless without a calculator when confronted with some matter involving two digit numbers.
We should also look at other things that impose a "high cost" on children: spending hours playing video games or noses constantly buried in mobile devices.