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

Peter Naur, "Programming as theory building." (1985)

“…programming properly should be regarded as an activity by which the programmers form or achieve a certain kind of insight, a theory, of the matters at hand. This suggestion is in contrast to what appears to be a more common notion, that programming should be regarded as a production of a program and certain other texts.”

http://pages.cs.wisc.edu/~remzi/Naur.pdf https://news.ycombinator.com/item?id=10833278 https://news.ycombinator.com/item?id=7491661