Due to the fact that they would give away the last piece of bread. Human nature is completely and exclusively egoistic. People are entirely selfish and devoid of any genuine feelings of sympathy, benevolence, or sociability. They are always thinking of themselves in everything they do. The object of the voluntary acts of every man is some good to himself.
Social organization originates out of self interest.
All society is for gain, or for glory. Instead it is for self preservation. It is a sort of social contract. In a state of nature we are at war with each other and life is solitary, poor, nasty, brutish, and short. In a natural state individuals are in equal powers.