Software Dev - Why Rebuild When We Can Retool?

basic70 asks: "There seems to be a strong preference for developing new systems all the time, instead of just refactoring and improving existing ones. Why is that? Modifications such as moving to a new operating system, modifying the business logic, adding a web interface, moving to Unicode etc, shouldn't affect more than perhaps 10-20% (to grab figures at random) of a decently built software system. I can think of two reasons myself. The first is that consulting firms make more money developing new systems, and the second that existing systems are so badly layered and modularized that any larger improvements are impossible. The second reason is scary, because that means that the modern way of building things with short lifespans is starting to make its way into the software business as well. I saw a system written in 1995 that couldn't handle the new millenium. Can't we do any better than that? The GNU suite says we can, so why is it so hard with commercial software?"