In contrast to software projects, traditional engineering relies on tried-and-true materials and methods, applied in well-understood ways to new applications. An aeronautical engineer may design a new aircraft that does not look or function exactly like any previous plane, but the structural members, outer skin, wiring components, and engines usually are copies of proven parts and techniques. Manufacturing plants, skyscrapers, roads, and bridges are built with a similar philosophy -- substantial reuse of known pieces, put together largely in known ways.
Traditional engineers, of course, sometimes advance the state of their art, by using new materials or techniques or both. The Burj Khalifa recently set a new height record for buildings. The engineers did this partly by employing a special formulation of concrete to achieve the weight-bearing strength required, and by pouring the concrete at night to aid in proper hardening. But these innovations are in tightly constrained situations, making incremental improvements to well-studied topics. Concrete has been around for thousands of years in some format and 200+ years in its modern version. New bridges similarly use known methods, with possibly some incremental innovations to solve special problems for that location. The public would not have it any other way.
Software projects often take the opposite approach, attempting to use many radical new materials (data structures) operated on by grossly unproven methods (programming code) to produce a machine that performs a never-before-seen function. Our large software projects often resemble precisely the sort of absurd dream invention described above. Why do we do this? The reason is that since we cannot see or touch software, it appears that all the parts of software, tested or novel, are quite similar and can be used equally well as machine subassemblies. In fact, software components do not all have the same approximate reliability, not by a long shot. We fail to appreciate fully that well-tested software modules can have a high probability of successful reuse, while novel software components are often a crap-shoot.
