Equality between members of a species is against the nature’s law of evolution because it is the survival of the fittest. Nature continuously weeds out and eliminate the unfit.
Despite the saints or preachers in every faith and all over the globe trying their best to persuade the human beings to be like brothers (equal), disparity is always there. Humans have divided themselves along the lines of region, religions and caste systems. That is why human beings cannot become equal.
In fact the desire for superiority over others propels the human progress because to be superior one must strive harder or be more creative. It is another matter that some of the people who become more powerful than others try to be good to others and also some powerful try to torment the hapless ones to become more powerful. The difference between animals and humans in this regard is that the weaker ones get eliminated in the physical struggle whereas in humans it is the mind games that kill the weaker mentally if not physically.