Paper trail of where it could have ended up maybe? And I mostly mean that if the testifying human isn't impartial, he would require physical evidence to corroborate his testimony, despite the oath.

The reason being sufficient examples and even game-theoretic arguments (mind you, these are 'nerds', they can find someone well-versed in game theory if needed) show a lack of (expected) trustworthiness of a non-corroborated testimony of such a human.