I have a two word answer: Intellectual Property. Not everyone is an accomplished Open Source publisher, nor should that be a requirement for employment. So what existing project do those candidates point the interviewer at? Someone else's IP? And then the hiring company is obligated in self-defense to make sure that it's not someone else's IP, lest it end up in someone's hard disk accidentally and get discovered in some otherwise unrelated audit? No. Thank. You. Keep your own code to yourself.
Now, you might point out your own code with some URLs and they may take a look at it. But that's not the same as making it part of the interview process.
I have all these same feelings about dev "testing" in interviews. Why does no one ask to go through an existing project that a person coded? I think someone's own explanation of software that person wrote -- including the good, the bad, and the ugly, is a much better demonstration of all kinds of coding skills (architecture, interfaces, documentation) than just live coding something or even a contrived take home test. Doing real work for a company without knowing their system is another artifice that doesn't really show you what someone can do when they know a system.
I have a two word answer: Intellectual Property. Not everyone is an accomplished Open Source publisher, nor should that be a requirement for employment. So what existing project do those candidates point the interviewer at? Someone else's IP? And then the hiring company is obligated in self-defense to make sure that it's not someone else's IP, lest it end up in someone's hard disk accidentally and get discovered in some otherwise unrelated audit? No. Thank. You. Keep your own code to yourself.
Now, you might point out your own code with some URLs and they may take a look at it. But that's not the same as making it part of the interview process.