The paradox that strikes me is that "hiring is broken" yet these companies are beyond successful. So there's still yet another layer of something in between observing that employees are capable / incapable, and the company successful / unsuccessful.
A personal tragedy of failing an interview has little effect on a multinational conglomerate. If the average employee is half decent, they will grow anyway...
Lots of humans. Lots of companies. Random chance.
Of course, its not that simple. Some companies probably are great at scouting. Yegge mentioned a few ways in the post. Good internship programs, acquihiring, etc.