Nigeria is not the wealthiest country in Africa, not even close. They have the highest GDP just because they have twice the population of the second most populous country in Africa. If you look at GDP per capita, they're barely scraping the top 20 of Africa [1] which, as you can expect, is not very high. It's growing, though.
Nigeria has a higher per capita than India, which is considered the #7 economy in the world.
What low per capita income tells me is that despite being a wealthy country citizens don't have a lot of opportunity to earn wealth. Which is my point.
[1] https://en.wikipedia.org/wiki/List_of_African_countries_by_G...