Personally , I think the two main reasons why Africa is poor are corruption and political instability.
Corruption and political instability go hand in hand and are at the root of poverty in poor countries, not just Africa. This is basic knowledge and the OP also made it clear that "corruption" is not an explanation he needs to know because it's something that no one doesn't know.
What he wanted to know was the underlying cause behind it all?If you ask me to find another reason, I can give two more reasons. First, lack of proper planning. And second, lack of attention from foreign countries.
We can look at the UAE as an example. This country has developed very quickly. Their leaders knew very well that Dubai does not have much oil. So they focused on the tourism sector. And they are successful because of proper planning. Most of the countries in the Middle East are dependent on oil money, but Dubai is different in this regard. This has been possible only because of proper leadership and planning. African countries are not getting proper leadership
If African countries had received sufficient investment from foreign countries, they could have developed. But foreign countries are not very interested in investing in Africa. This can also be considered one of the reasons for their backwardness.