There certainly are those who are self-interested, but there are many more who really do care about the public good. You hear about the ones doing backdoor deals and shady stuff like that on the news because it makes a more interesting story than someone who goes to work each day trying to make their city a little nicer, or pay teachers a bit more, or cut down on crime, etc.
You can't do much public good if you don't get re-elected. Since public opinion suggests people want more services and less taxes, something is going to have to give.