Because there are so many other corrupt nations in other continents for one. But also since natural resources are the primary and virtually entire means of income for many African countries - the resources have been able to remain largely in control of those generals and rulers who are willing to play ball with the purchasers of their cheap natural resources. This unfair balance in the trade relationship has been the status quo ever since the end of European colonialism which hasn't been for very long in some nations. It is what is sometimes called The Dependence Theory: Industrialized nations profiting from exploiting non-democratic, therefore low wage-paying governments who in turn become dependent on the business of industrialized nations to stay afloat. So interests in the West predominately still have major influence on and profit to gain from the propping up of so many tyrants and corrupt people in positions of power.