Writing well is hard. From what I've seen of Martin, he has a general outline, maybe a few characters/scenes he can get passionate about and really write those well, then he tries to motivate himself to write the less interesting shit that ties it together. When he can't self motivate, he does side projects on stuff he feels motivated about. When you aren't motivated, you also tend to be highly critical of your own work and probably burn through tons of "junk" drafts.