Here's a shot at an answer, lifted from 'The Mythical Man Month', Fred Brooks: "Software entities are more complex for their size than perhaps any other human

Here's a shot at an answer, lifted from 'The Mythical Man Month', Fred Brooks: "Software entities are more complex for their size than perhaps any other human construct, because no two parts are alike (at least above the statement level). If they are, we make the two similar parts into one, a subroutine, open or closed. In this respect, software systems differ profoundly from computers, buildings, or automobiles, where repeated elements abound."