Because in all too many companies, re-writing from scratch is a no-go, no matter how quickly and sloppily an initial solution was thrown together. I've worked on a prototype => production type project, where the throwaway was never thrown away. (the initial team made some mistakes, chief among them was building one prototype of a whole system, rather than one per major risk)
This is a systemic problem. Engineering is always subordinate to business. This simply should not be the case. We desperately need new business organization models.
