This answer is facile, but missing: It takes so long to build software because it can. One can't construct a building forever. A novel is published and done.

This answer is facile, but missing: It takes so long to build software because it can. One can't construct a building forever. A novel is published and done. But software can be developed over its useful lifetime. Similar followup: The parts that take a long time, do so because the quick parts were finished quickly. Lengthy software development isn't a problem with software, but rather reflects the frictionless malleability of code. "Why does software dev take so long," isn't the same question as, "How to develop software more quickly?"