The problem is GRRM develops the story as he writes. Yes, he has a general outline, but he creates as he writes, and he doesn't control the branching narratives well, and now he is overwhelemed and can't finish.Some probably do, and he probably did at one point, but if he actually enjoyed doing it, don't you think he would?
There are some writers, it seems, whose goal is not to write the best book they can, or even write a best selling book, except insomuch as those goals serve the ultimate goal of getting an adaptation made, because that's where the real money comes from.
Authors like Brandon Sanderson map out the entire story first, and write the story to fit the outline. It is why he can publish a book or more per year.