So who do you pick? You have all these well-educated, similar people. So of course you go for the ones you like. You ask them stupid brainteasers and stock questions (what's your greatest weakness? Where do you see yourself in 5 years?) and just see who will say something you like. They are just as capable as the ones you don't like. This is also the reason you need to have gone to a top uni: why bother looking for a skill that everyone has? Nobody ever got fired for hiring IBM.
Oh, one exception: the children of the powerful. A buddy of mine worked in investment banking and there were a number of kids whose parents were CEOs of known firms, or other important people. Those kids are going to be able to do the job at hand as well as the future job of getting business. It does tighten the pyramid for the other capable kids, but they know the game.
Contrast that with a job where the skill is roughly as available as the demand. Where it is possible that the random guy off the street cannot do it (eg iOS developer). What do you do then? Well, now you can't just show up to the interview unprepared. You need to spend time asking real, discriminating questions (How do optionals work in Swift?). You also might need to balance your opinion about the person's character with your need to have that skill in house.