It's a lot easier to verify the process. It's a lot easier to check that all the drives leaving the building are physically destroyed, than it is to verify that they contain no sensitive data.
a little off topic but the mob has been operating this way for years when it comes to risk management... dead men don't talk, no matter how trustworthy the individual might be, it's easier to put a bullet in someones head and be done with it, than to worry about what could happen later.