The funny thing is, he's been working on 6 books over a span of 20 years.
Compare that to Terry Pratchett: 22 Discworld novels in the same timeframe, all in the same universe, all hanging together, including hanging together with the previous 18 novels he had written over the previous 13 years. And that's with him having Early Onset Alzheimer's which has caused his writing to slow considerably in the last 8 years.
Sure, they write in different sub-genres, but Martin could easily have broken his novels up in this manner as well, but chose not to.