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.
This isn't really unique to capitalism - it's just politics and interpersonal competition.
This kind of thing happened in the Soviet Union, it happened in monarchist France (court of Louis XIV), it happened throughout all of human history.
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.
But I expected it to be US, frankly. Didn't expect "personality tests" from UK.
That's because they're not sharing wins from stellar candidates but they take blame for bad ones.
I wonder why blame shifting seems so central to american way of thinking. You'll give talent away to avoid it.