Hm. A few thoughts...
A writer and a storyteller are not quite the same thing. Writing fiction is telling a story in one particular format, with it's own peculiarities. Being able to write a good tale does not equate to being able to tell a good tale, face to face.
There's more than one way to skin a cat. Mr. King finds that, for him, plotting isnt the way to go. That's cool. For other folks, plotting may work just fine. Since the reader isn't involved in the process of creating the work, how you get to the end point is irrelevant, so long as it works for you.
RPGs are not written fiction, in that the audience is not only privy to the process of creating the story, but they are involved in the process. Much of what goes on in the story never gets spoken by anyone at the table - it goes on inside the heads of the players individually.
Which brings us to GMs as writers/storytellers. GMs are, like authors, telling a story in a very specific medium, with it's own quirks. A good GM makes shameless use of those quirks. But being good at using them doesn't make him good at telling stories in other formats.
F'rex - a GM doesn't have to do nearly the amount of colorful descriptive work that a writer does. While he does some description, most of the blanks get filled in silently by the player's own imaginations. A good GM does the minimum description required to fire the player's imagination, but no more. A writer who does the same will find his reader has much less grasp of the physical world in which the story takes place.
F'rex - an author has to decide the result of every conflict in the story by fiat. The GM generally doesn't decide any of the results - he allows player action and the dice to do that work for him.