It's a pretty complicated issue and it involves colonialism.
Basically, most of those African countries only gained independence from colonial powers since the 1950s. While under colonial rule, the foreign countries exploited the local resources to enrich the primary country, not to develop the African one. Additionally, even when the colonial powers pulled out, their companies remained in control of many of those resources so even though the African countries were independent, they didn't control the resources that could enrich them. On top of that, the local population was never taught, trained, etc. on how to take advantage of the resources so they remained dependent on foreign expertise until they can develop sufficient homegrown talent.
Also relevant is the national borders themselves. Many of the borders in those countries were created by the colonial powers, not the Africans themselves. One negative consequence of such a system is that groups that had no affinity for each other and often had animosity towards each other were suddenly forced to co-govern a "nation" before they'd even finished settling their own differences.
Sorting out all of those things takes quite a bit of time. People have to manage their inter-community differences, organize and run a government, find out how to regain control of their natural resources while not having been given the opportunity to learn the skills necessary to maximize those resources and simultaneously catch up to the rest of the global economy on the fly.
It's going to take more than 50 years to get through. A far more instructive area would be to examine the growth from date of independence to now on a country by country basis, factoring in the things I've mentioned and then separating out which countries have made tremendous progress vs. which ones have not and what leads to those different outcomes.