I see your point. But the key here is choice. You chose to go to the gym and do repetitive exercises. You probably do this in part because you want to be healthy and it makes you feel good. Do some kids want to do repetitive math exercises? Sure, I was one them! But they are a small minority. Should we force kids do repetitive exercises just because it is good for them? It's a tricky question.
Likewise with math (and many other disciplines), there is a core set of skills and patterns that need to be mastered - once you truly understand those skills and patterns - you can then use them in creative ways to solve the problems.
Khan's great skill is in helping to explain those skills and patterns, leaving the teacher time to help the student master the practice of them.