Memes

A big part of the "selfishness is human nature" thought process comes from Puritan brainworms. The Protestant work ethic and the belief that one's desire to have the basic necessities of life is inherently selfish form the basis of American capitalism, thus the idea that one must be worked beyond their limits in order to simply continue to live becomes baked into the fabric of society.

Basically everything wrong with America is John Calvin's fault at its source.