Your car buying analogy is flawed. When you buy a new car, someone has built it for you. It's cost effective because the manufacturer builds a great number of t

Your car buying analogy is flawed. When you buy a new car, someone has built it for you. It's cost effective because the manufacturer builds a great number of them. You can be fairly certain that it works and if it doesn't you'll have a guarantee. When you rewrite a software system, you do it yourself. You don't know whether you'll succeed. You might end up with worse end-results. The assumption here is that no off-the-shelf software can be used to replace it. Hence rewrite.