1. They have.
2. Sub-Saharan Africa doesn't have the required plants & animals that are needed to jump from a hunter gather society to an agricultural one.
If you stuck stone age White people in Africa they would likely be in a similar state. 
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A lot of these, mostly the most noteworthy civilizations ie Egypt & Carthage were actually built by the Arabic and mediteranian peoples, not black Africans.
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Then I guess black *and* middle-eastern people are both vindicated by this list
End of conversation
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