I can't speak for anyone else, but I don't need a supernatural being, or the accompanying religious leaders to dictate, cajole, or threaten in order to be as good a person as I can. Laziness does most of it. I don't have the energy for confrontation, lying, stealing, or violence, or the physical or emotional distress they cause. I like things to go smoothly (they don't, but it would be nice), and have plenty of time to explore all the cool stuff in the world. 

It's sort of like avoiding bears. Behaving badly is like deliberately looking for a bear, and that rarely ends well.