I've recently been reading books about athletes. Lance Armstrong's "It's Not About the Bike," Andre Agassi's "Open," and more recently, Scott Jurek's "Eat and Run." All these books are memoirs, but they have something less obvious in common. They all had ghostwriters.
The reason they do is simple: Writing is a skill. To become good at it requires training and discipline, just like sports do.
To write a bestseller requires talent as well, just like making it to the top in sports does. While some people may be gifted and can write well without any formal education, it's extremely rare. As rare as the person who goes out and wins races or tennis games without any formal training. It's not that it can't be done, it's that most people get there the hard way — through pure self-discipline, experience and extraordinary talent.