This question should be directed to the African Nations. And in my own understanding it is because of the greediness and corrupt nature of the leaders because a good leader knows when to provide jobs for it citizens.
Not only in africa nation we have experience unemployment, some countries experience unemployment, if it's well with other continent why is that economy inflation is also affecting some of the other countries...they're people who choose not to work, in south africa, theirs employment opportunity but some of her citizens choose not to work
If the population increase, as a leader, you can also expand the industries to accommodate the work force. Africa has a lot of things to do but here is no industry to work. And those are some of the reasons unemployment is very high. The money that supposed used to establish work for the people is siphon by the politicians.
In good country their's a lot industries were people we work, it's not only governments that provide employment opportunities in a country, a private sector also provides employment opportunities, that's why in some countries governments empower some entrepreneurs to establish more, so that they will absorb some individuals into their companies and also help to reduce the rate of unemployment in the society.