For the same reason any similar arrangement holds: if it gets bad enough, the torches and pitchforks come out and the group finds themselves a new negotiator.

> why won't those few people negotiate better deals for themselves while keeping peanuts for the rest?

Because they would be very bad negotiators and wouldn't be hired in the first place.

And because they probably are scamming the rest of the group, which is a crime in most developed countries.