There are lots of possible answers. It could be that intelligent life is pretty rare, so that even if there was a species out there building dyson spheres or doing other crazy things in a far distant star system we wouldn't be able to detect them. Or maybe there is a sociological explanation, like civilizations tend to be unstable and collapse in on themselves before they can become type 1.5 civilizations. Or maybe most intelligent species are trapped in their solar systems or their home planet, that interstellar colonization will never be economically feasible.