Peter Naur's classic 1985 essay "Programming as Theory Building" argues that this is because a program is not its source code. A program is a shared mental construct (he uses the word theory) that lives in the minds of the people who work on it. If you lose the people, you lose the program. The code is merely a written representation of the program, and it's lossy, so you can't reconstruct a program from its code.
https://news.ycombinator.com/item?id=10833278
https://news.ycombinator.com/item?id=7491661