Haven't had one of these yet - but will count myself lucky.
I'm going out on a limb here and say that a huge part of the problem is companies just treat the interview process as something anyone can just "do". No direction, no training, just hand someone a resume and say "go!". I can't count the number of times I've had to run the interview from the wrong side of the table b/c the interviewer had zero clue as to the role or even how to ask basic questions. It's all based on emotion and feelings...
I did work at a company for a few years that did have a very rigerous and purposeful process. Each step had meaning and we would apprentice for sometimes months before being permitted to conduct a particular step on our own. I loved it and we were pretty successful at identifying good fits for both skills and culture.