Because blame goes down, and reward goes up, so its a lot more important to make sure that the blame for wrong doesn't get to you than to make sure you get the credit for right, since the reward for right is an "attaboy" where those above you reap the substantive benefits, while the punishment for wrong remains substantive.

TL;DR Because capitalism, mostly.

It often seems like hiring stellar candidates involves circumventing HR policy. HR responds, instead of asking how they can help be part of the solution in the future, by incorporating new policy. Hiring stellar candidates, therefore, creates a situation where it makes it harder to hire stellar candidates in the future. There is no "win" in that game.

FWIW, the story the OP told took place in the UK.