Fantasies and myths. When you look at an architect's drawings for a house you can tell exactly how much wood is needed to produce the necessary number of cripple studs, wall studs, jack studs, king studs, bottom plates, top plates, headers, trusses or rafters, etc. And any good framing carpenter can tell you how long it will take to build a wall. But we've only been doing software as we know it today for 50 years or so. We're not that good at it yet.
Software is still largely a creative act.
And there is nobody on this rock who can predict or schedule a creative act. Anyone who claims they can is selling something that should be put in a flower bed. Well, here are some recent examples of engineering decisions that were not made by engineers. I think that pretty much says it all. What's even scarier is that I'm allowing this to continue.