"It seems that you are saying that learning by doing is better than learning and then doing. If so, why does the average college graduate earn about $1 million more in his/her lifetime than the average high school graduate?"
In college how do you think they learn? by doing. Besides that I'm willing to bet that outside of some professions (doctors, lawyers and some others), most grads don't use their degree for the profession they majored in (don't take me wrong I still regard the college experience as valuable and I hold a degree myself).
As far as sales goes, many people may learn the hard way, but they do learn.