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@Milano Ahmed said, colonialism really messed up Africa, a large part being that these nations now have borders that someone else drew.
You often hear that colonizing powers drew borders with no care for what people happened to live within them, but that's not true.
Frequently, borders were drawn with the intention of including rival tribes or demographic groups.
Playing them off against each other, for example, by designating the smaller group as the ruling class and the larger one as the laborers, was a great way to keep the nation in line. It guaranteed that the ruling class would be reliant on the colonial power to prevent uprisings, and so they would do all they could to please the colonial power, often worsening relations between the two native groups in the process.
And of course another significant reason being power itself, Power attracts corrupt people, and once those corrupt people have that power, they are not interested in giving it up. They carefully surround themselves with people who help them maintain that power, and be sure that the institutions that protect their power (i.e. the army) is compensated well enough to make it worth their while to keep those power structures in tact. No country is actually immune to this. But you don't see the more extreme cases in the likes of Japan compared to Congo because they generally have a more educated populace that aren't easily swayed.
