I honestly think he hasn’t finished the books because he’s got no idea himself how to actually end it. And after the backlash the show got, he’s now pretty sure it shouldn’t end like that…
His two main assistants also fucked off and then wrote The Expanse series of books. Style is surprisingly similar to GoT, and makes me wonder how much was George and how much was them…
Then The Expanse got made into a show and thems two fucked off for good.
IMO his assistants did a lotmof the heavy lifting, and there was too much left to tie off with GoT so he just gave up.