I think by this point - or really even a decade ago already - we have to conclude that, as great as the first three books were, GRRM just doesn't know how to get to an endgame and a climax that are anywhere near as good as the start was. Nor would I or as far as I know any other fan, to be fair.

Whereas it's much easier and more fun for him to flesh out the worldbuilding and the historical background. In other words, I don't really think he'd be making any more progress with the main series if he wasn't spending any time on the prequels. He might as well work on stuff that he does know how to write and which fans also like to read.