Because each discrete component that you can eliminate (and replace with code) is a weight saving. Because once you tip over a point in complexity, you just kee

Because each discrete component that you can eliminate (and replace with code) is a weight saving. Because once you tip over a point in complexity, you just keep adding more code to guard against more edge cases - edge cases you can't avoid because they involve crashing into mountains. Because you want to offload as much possible effort from the cockpit crew, while still allowing them full control over the automated feature All of these things just add more and more code.