Why aren't ducks and tigers best friends? Its all because, though we are the same, we are also all different, which can be good or bad. Every race believes their own things, same with religion. To agree on one, or try to put them all together, when many of them conflict anyway, is like saying that only one is right, and the others or wrong,-sorry everyone else, you have to give it up and accept it, too bad. And no one wants to do that. Besides, how would we choose a final life style to follow, without anyone giving up their beliefs.
Though it is a great dream, I don't believe it is in any of our powers now to do such a thing. And that's just the way it is.