Because it will lead to an incomprehensible mess. Ever heard the quote, "Programs are meant to be read by humans and only incidentally for computers to execute"? This is well-trodden ground in science fiction. If you have AI writing code that's so lacking in abstraction (because machines require less of it to understand) then humans will become useless in maintaining it. Obviously this is a problem because it centralizes responsibility of maintenance onto machines who depend on this very code to operate.