Why? Many times salespeople aren't capable of managing themselves, let alone others. Often, great salespeople are not well organized, but they have great people skills. That's why there is nothing wrong with salespeople remaining in the field their whole careers.
Managing one's self is the most important job any salesperson will ever undertake. In learning this discipline, you will find that the job opportunities open up and your production is enhanced. Most salespeople are self-starters and must do their own coordination. Don't wait for things to happen - make them happen.