Why are people so rude to those who work in fast food or retail?

So, I work in fast food (college student here) and have typically noticed that customers (usually the elderly or suburban moms) tend to get SO angry when something doesn't go their way. I get it, things happen, people have bad days, and sometimes, it is our fault and they have a right to be angry. But it is so frustrating when our store is SUPER understaffed and people think they can just bitch and complain about every little thing. I understand it sometimes, but most of the time I think they are complaining about things that don't even matter. Why do people think they are so entitled?