Because a lot of people really need a job, get very stressed at the thought of not getting a job, and are then paraded across an HR circus doing things they don't understand why (just because you have rationalised this group selection process doesn't mean someone new to it understands what they hell you are trying to demonstrate to your interviewer by building straw towers). They try to do their best, taking it seriously and end up failing because they weren't the happy-chappy type of person that you are looking for. Basically, failing because they were too dilligent.
Any good interviewer should be able to figure out whether someone has customer facing skills through a quick interview, you don't need to make people feel uncomfortable.
There are scores of people applying for such jobs that would have no problem working in that particular environment, and be great at it, but "you" have structured a whole bizzarre recruitment act to figure out who is the best - instead of just talking to them.
The whole and sole purpose of the group interviews is because it's easy to filter through multiple candidates using few HR people at a time, a task that would otherwise take up a lot of resources to do on a face-to-face basis. All other excuses are just facitious bulldung. In the process you have allowed some HR muppets to go ballistic with their psychological experimental theoritical mumbo jambo on how to "read" candidates and extract the information they want, anything but talking to them like normal people!
btw, I've never applied for such a job so dont think I hold a grudge or something.