To me the difficulty of estimating software projects isn't about estimating the time / effort to build the thing. The biggest uncertainty is trying to estimate

To me the difficulty of estimating software projects isn't about estimating the time / effort to build the thing. The biggest uncertainty is trying to estimate what it is we're trying to build in the first place. i.e. the details of the spec. Often times where I've seen estimates really off is when the scope changes, or more details are discovered etc. To borrow the usual building a house analogy, if during software estimation we're actually given how many rooms, how many floors, where are the doors and windows etc, then I think estimates will be a hell of a lot more accurate. Instead, what we typically get is, build a house that can be used by the family of 4, and we'll discover exactly what the family wants as we build.