It's just that cooperation, exchange, and division of labour benefit all of us better than some zero-sum game. If this were not the case, savagery would rule the world, instead of global trade.
Enlightened self-interest favours respect of diversity and non-conformance - because exploration is more effective when everyone tries a different approach (instead of the same one). And exploration, especially in science and technology, benefits us all, particularly in increasing carrying capacity so we needn't choose between intertribal warfare, infanticide and starvation.
I took the following from studying world war two in school (though I don't think it was explicitly stated): the Nazis started WWII and sought to eliminate many groups, including Jews, on the basis of eugenics (apparently, eugenics was broadly popular at the time). Yet Einstein, a German Jew, created the basis of atomic weapons which ended the war (if not, they would have ended it).
Eugenics is a misunderstanding of the Darwinian value of a human life.
To some people, this point of view may seem cynical, ugly and horrible. But to me, it is the most hopeful view I've ever seen. It means that if some overly-controlling group tries to enslave the world, they will likely fail - because their very nature weakens them. It also means that over time, the Darwinian pressure is for us to become ever more good, as that is what gets rewarded. Therefore, if one day we meet powerful aliens, they are likely ethical aliens. Taken to the furthest extreme, one could see symbiosis as the nature of the universe.
Is that taking it too far? Maybe. But it follows from the evidence, and the logic of division of labour. Of course, it's just a probability - selfishness can strike at cooperation at any time. And does. All I claim is that cooperation is the way to bet.
Ethics is power.