Why are people so self centered and unreasonable nowadays?

I work in retail and like most stores, we are understaffed and overworked. I work 6 or 7 hour shift with no break, no lunch, and no there isn't one person that can cover for me because every area is busy and understaffed. I work in a customer service department which gets busy., I am trying to do orders, make phone calls, wait on customers, do projects, etc and sometimes I will have 6 people in line and do to the fact that I am only one person, I can't take care of everyone at once. I try first come first serve or if someone has a small request I will take them first to get them on their way because I don't want to make a person wait to pay for one item while I spend 30 minutes with another person making a custom order. So many people get impatient and holler at me and all I can tell them is sorry I will be with you as soon as possible but this person has been waiting and was here first , etc. Why do consumers blame the employee when it isn't their fault?