That is the natural course unfortunately. It doesn't "have" to be that way, but it presupposes strict self-control and shared morals/ethics. For example, it is in your immediate best interest to vote yourself favors/money, but ethically you shouldn't as it would be wrong to take from others or tell them how to live.

This standard, can persist, but it is such a high standard for the masses and breaking it can be done so privately and carried out through a 3rd party so that history has shown this standard always falls, without fail, given enough time.