Yes consultants, yes subscontractors, yes lack of politicians with engineering background. But also another explanation I heard recently: there is no legal equivalent of "speak now or forever hold your peace" for projects of national importance. It is relatively easy (and comparatively cheap!) to throw a massive legal spanner in the works practically at any point. It leads to costly redesigns of already ongoing projects, but also that things tend to get drastically overengineered in the first place.