On the other hand, the average distance between cars on the road vs. planes in the air is quite different.
If a solo student pilot in a plane makes a fatal error, it is very, very unlikely that he kills somebody else while doing so. But if a solo student driver in a car makes a fatal error, chances are very high that he takes somebody out with him.
In all types of learning, we makes mistakes, and learn from those mistakes. However, in an environment where making mistakes can easily lead to the death of others, you would not want somebody to make those mistakes without somebody else nearby who can instantly correct those mistakes.