And yet they often do. Consider that for a moment and wonder why it is so common, especially in technical companies.
What I have found over the years is that technical people hire people they think are great without regard to the HR "signals" and then one day they get screwed. Maybe they hire someone with anger management issues, maybe they hire someone whose antics expose the entire company to a crushing lawsuit. Or many they hire someone who, in short order, irritates all of the other employees such that there is a huge morale crash and exodus. Basically their hiring on technical merits and/or interview results in a very bad outcome. And then someone they know and respect says "Gee, an HR person would have spotted that right away, why didn't you listen to them?" or worse "Gee if my HR person had let that person through I would fire them on the spot."
You see in both cases something a bright technical person might be actually unable to see, could be the difference between a "good hire" and a "bad hire." And that is how HR people get into positions of power over hires and fires.
1) The hiring manager has someone else to fire (and blame) when a hire goes badly.
2) The hiring manager has some 'cover' over the things they don't readily see (like emotional issues).
Of course since the whole emotional/psychology thing is so opaque to some folks it is really hard to judge if the person they have providing that visibility is good or not. Sort of like someone who knows nothing about technology hiring a consultant, or someone who knows nothing about cars hiring an auto mechanic. One has to take things on faith a bit that the other person knows what they are doing and try to come up with ways to re-assure yourself that this is true.