Population Control

Because simply "Making a country wealthier" is short, simplistic, and doesn't actually solve anything.

To give a person a fighting chance in a culture ravaged by warlords and famine you have to give a person much more than material wealth. You need to give them food, shelter, clothing, and education. And it's very difficult to give them these things when its in a warlord's position to make sure that these people do not ever get them.

Also, it takes several generations to make a marked improvement in the wealth of a country. In these days in Africa, children are born to malnourished mothers and the children live malnourished. This makes them have irreversible damage when it comes to cognitive function and physical strength regardless of how much you feed them. The actual cost to give these people some amount of fighting power in their communities is astronomical when you think about everything you have to give them over a period of years and years. And that's assuming that the warlords or oppressive regimes will let you do it.

So, good luck giving people in Africa your money. Simply giving them money won't do anything. And that's a damn shame.