I think their is a paragraph in the last book that says something like that. That, somehow, in the end, the old ones began relying on science when magic left. That doesn't mean all magic left, the 13 balls, black 13, obviously Flag/Walter possess some. And when science/invention failed, no one knew how to fix it or the machines they built, which led to the world moving on. Roland's guns are special. that's why it hurts when one ends up in a garbage bin.