He could have written book 6 and 7 any time while the HBO series was being released or not have agreed to the HBO series at all. He certainly had plenty of time to write all his Westeros fake histories which he chose to work on instead of book 6 and 7 of ASOIF. It is what it is. George got his big payday with the HBO series and is going to just write worldbuilding bloat fake histories until his heart’s content.