Why are leaders in Africa so corrupt?

Asia use to be like Africa,but their leaders started open themselves up to international trade--and as a result they are very successful. Why! in countries like Zimbabwe, did the leadership cut themselves of from International trade, and cease white owned farms, which of course, turned a bread baket into a third world country. And, why in South Africa, are the whites and the blacks still mad at one another. So my point is, why hasn't the leadership in these African countries open themselves up to International trade like China, Japan, Thailand did. Are they so greedy, that they only care about themselves.