Because code does not include the thought processes that went into creating the final code. Take a second and have a look at the Linux kernel code base and get into that. It's surprising how some code only make sense if you understand the bigger picture.

So it will be with AI code that has just been generated and blindly added to the code base. It makes everything work but sometimes, perhaps not always, the devil lies in the details.

Take any book, open it up to a random middle section, read it. I can read the words but I don't understand the story. And so it is with code.