Actually, strong, nationalistic military leaders have been. There're, of course, corrupt, authoritarian leaders that decimated their countries, but those are not unique to Africa.
Muammar Gaddafi was of that strong, nationalistic military type who was very good for his country, that is, until we decided he was too much of an impediment to our interests.
You should look at the average, not cherry-picked benefits. Sure, every continent has had authoritarian leaders who did some good.
But the average dictator just exploits their population to enrich themselves and sets the country's development back substantially. (That being said, I don't think the US should be getting involved at all. Establishing good governance is Africans' own responsibility.)
Muammar Gaddafi was of that strong, nationalistic military type who was very good for his country, that is, until we decided he was too much of an impediment to our interests.