Because one might have had a better view of the play than others. I don't know what you're suggesting, but if it's that the refs shouldn't be allowed to get together and discuss what exactly happened, it's not a good suggestion. I'm guessing, but don't know for sure, that the ultimate authority to determine what the call on the field is belongs to the referee, but he has to have input from the other officials. Otherwise, what purpose do they serve?