I blame the gate attendants, some of them anyway. Some are very effective in communicating which group they're boarding, and asking would-be gate crashers to step aside, while others get on the microphone and go through the routine, but they then let everyone crowd up and take every boarding pass that's handed to them. I suppose that although they know they have to go through the boarding-by-group spiel, all they really want to do is get everyone on the plane as quickly as possible.

When I fly ORD-SCL on AA, the boarding by group goes smoothly, but on the return, the AA gate attendants at SCL rarely make an effort to board the groups in order, with the result that everyone crowds together and elbows their way to the front of the line no matter what group they're in.