.. is that it's soft. Malleable, pliable, moveable... wobbly. And, what's more, invisible. This makes it much harder to engineer than, say, a chair or a car.
With physical objects, you get predictable results: the chair stands, the car starts. Even when things go wrong, the symptoms are relatively straightforward, and the causes clear: the car won't start because the starter motor is broken or the battery is flat, probably caused by wear & tear or unusual weather.
This article (BCS) argues that software should be designed based on this dependability: risk-based software design. It's an admirable principle, but where to start...?