It's sort of like saying, "Why can't you just clean this enormously messy code base up one bug at a time?" Sure, you can in theory. But it's a very rare project that does it. Even places that do total rewrites often eventually end up with as big a mess as they had before. Why? Because the real problem is the ideas, relationships, and habits that got them the messy code base in the first place. That is extremely hard to change.