I can answer for the people I know personally. It sucks to be from a broken system / useless country, where even basic things don’t work / work well. This especially gets worse after some exposure to working systems, and many people develop a complex to fix stuff where they’re from.
Sometimes, it’s just plain wanting to fix a problem that you really need solved for yourself. Why can’t I do X in my country? This means so many people can’t do X, so maybe I should build Y to provide X. From an impact point of view, it makes sense too, I guess. Local champions are champions regardless.
I don’t know what startups East Africans are building, but a good number of Nigerian startups solve basic problems.