Why is it so hard for some people to just give credit where it's due?

Despite this guy designing an automation program that would save the company hundreds of man-hours, the boss kept giving him the runaround on actually deploying it... at least until he could steal the credit for it and earn himself an undeserved promotion. What he wasn't counting on, however, was that this employee had already anticipated such a contingency and still had one last card to play on his way out.