Because unless they're being watched, the strong can take what they want. In a perfect world they wouldn't do that, but they do, and why would they let go of this power?
On a global scale it's the US and China that's the big strong warlords, in Africa it's local warlords. Unless you can change our animalistic desire to have things, and get them the easiest way possible; then your perfect world will never happen. I believe it'll never happen.
Unless we discover an alien species and decides to wage war on them I see no reason for things on earth to change. Besides, even if everyone decides to play nice; should less developed countries be denied the luxuries we in the west take for granted? Constantly brand new PCs, phones, gaming consoles, high speed fiber optic cables connecting us to to the world wide web, high standard cars, more clothing and food than we can ever consume, all made dirt cheap from slaves down in a country we can't even pronounce.
First of all there's not enough raw materials in the world for that. Second of all, like hell the current slave workers will accept working for shit wager with a shit job.
Or did I misunderstand, did you mean we should have world peace, but still have the world wealth gathered in a minority of the countries, and have the rest of the countries work as slaves who only lives to provide us more things to consume?