Because in their minds, they are aggrieved and thus entitled. See these attitudes all the time. Kind of ran into it today at Fresh Market as I was buying lunch. Two women took forever at the register because: (1) they were talking to a third friend on the phone rather than unloading their carts and getting ready to pay; and (2) they had to split their cart between them and make two separate payments (and then the second lady proceeded to document her payment in writing in her checkbook).

Now, in her defense, the second lady did look at me and sincerely apologize for holding things up before walking away. She thanked me for my patience. Which completely evaporated my frustration.