His inspiration is so far gone that doing anything else has become a defense mechanism at this point. Imagine the worst period of procrastination you've ever experienced. It has to be like that. His heart isn't in it and the task of getting back in and tying everything up in a meaningful way probably feels insurmountable, but the immense pride that comes with authoring ASOIAF prevents him from passing the work on.

A lot of people think Brandon Sanderson should finish the series but I think that would make for a very stale and sterile end to the series. Sanderson books read like a movie script or something. They just keep moving forward without really slowing down to paint the environment and let the story breathe. It has to be someone else.