Why do people expect excellent/perfect customer service?

Like from working in food service, it seems like people won't be satisfied unless you give them top notch customer service. Like why do people come in to places expecting to be bowed down to and if you mess up one thing that they are looking for, they want to complain? I noticed today that people like for every little thing and get all pissy if they don't have their way for everything. People are human, they make mistakes and aren't mind readers. Why do customers have to expect way too much out of the employees? I mean if they put customers first, why does anything else matter? Don't people have more important matters in life than to worry about how well they were treated in food service places?