In 2 Broke Girls Max and Caroline work at the diner and they do extra jobs (selling cupcakes and other stuff). So they have two salaries, but even then they can't afford toilet paper! How is that possible?
I'm not sure of where they are supposed to be located, but you could probably assume a large city in the northeastern US, probably NYC (they may say within the series, I just don't remember). Cost of living is not cheap in big cities, even with two jobs. With that said, first, they are only working as waitresses in a greasy spoon, so they are not getting paid a lot, and I'm sure tips are even less. Second, they are trying save money so as to open/operate a cup cake store. They don't have any money because they don't have much in the first place and they are trying to scrimp and save what they do have the for the store.
Short and simple: many jokes are based on the problems of the different characters. In other words there has to be a reason to laugh at. And one reason could be other peoples problems.
The girls have these problems because the authors want to. Maybe next episode Max buys an island in the caribbean for one dollar on ebay or Caroline wins two million dollars in the lottery. The situations are pure fictional.