We have seen a lot of egalitarian theories trying to construct just practice in human society. They filed. Why?
Because we tend to forget the basic traits of human nature. Selfishness is one of them. It takes 25 or more years of socialization to "train" good member of society, many times this effort fail, too.
It seems that this is unsolvable problem of social dynamics, which can be meliorated on in some types of society. And, we are still looking for best of all possible worlds.