African countries are some of the poorest on the planet. It takes a very large capital investment, in addition to skilled labor, to energize a nation. They often have other bigger issues to deal with. It's often way cheaper to pay for humans to mine coal, produce charcoal, extract oil (not uncommonly a private company - or a country like China - extracting the resources and making a big profit, or giving a loan with a huge interest payment).

But let's look at a single off-grid DIY example. First you need the expertise to hook it up. If you have the skill, it still isn't cheap if your economy is weak. It requires saving money for a capital purchase, something people all around the world struggle with. And when there are payment programs, they're often exploitative.

It's not like people in Africa aren't aware solar power exists. If it was cheap and easy, everyone would have done it already.