I think he is busy making sure AI doesn't finish it first. Can't have AIs trample in his fantasy land.

He could have finished the series long before the Game of Thrones adaptation was in full swing, much less the general availability of LLMs. I think the HBO money made him care a lot less about ASoIaF mainline and went back to editing Wild Cards and other projects.

LLMs are really bad at worldbuilding outside of tropes. They're great at coming up with on the fly setpieces etc. halfway through a session, but for novel concepts they really dont work that well