I've been telling people this since somewhere around 2014/2015 when I read the third book. The first two books were well thought out, the plot moved, the exposition had purpose and was driving toward something. While I was about 2/3 of the way through the book I realized that it felt like GRRM had changed his mind about what he wanted to do with the story. The book no longer seemed focused on a destination, it seemed focused on moving characters around so that he could make something different work instead. But doing that new thing meant killing off 75% of the characters he'd spent two books developing, so he had to replace them with new ones, who were less developed, kind of cardboard cutouts of the previous ones. But now these new characters stories needed to be fleshed out so he could make their involvement make sense. In doing that he realized he couldn't slot them in to accomplish the goals he needed to complete the story. So he kept expanding the web, expanding the universe, but never really having a plan or path in place to make it all come back together. And that's where he's been for over a decade.

He hasn't finished the books because he doesn't know how to at this point. He can't get everything tied together, he can't go back to the story he wanted to tell because he killed off pieces necessary to make it happen, and the replacements didn't fit where he needed them to.