Some of those story lines are epic. Perhaps the best I've seen is the "Untitled 2008 Christmas special" which was "On Christmas Eve, an alien creature attaches itself to author J.K. Rowling. Suddenly, the real world is replaced by a magical reality influenced by the writer's own imagination. The Doctor must battle witches and wizards to reach Rowling and put the world to rights."
Why didn't they do it? Can you imagine The Doctor vs Voldemort?!