Providing problems automatically on a website like Khan Academy is technically very challenging, because problems are hard to write. Currently, the many exercises are generated automatically with random numbers every time but such a scheme is impossible with harder, deeper problems – instead the problems need to be written thoughtfully and carefully, something that is hard to do on a large scale. In addition, answers to problems are usually significantly harder to verify than answers to exercises and so would be hard to implement on a large scale like Khan Academy is trying to do.

Like you point out in this post, problems are probably best left (for now) to individual teachers who can spend more time and effort on individual students than is currently possible on a larger scale.