Africa as a continent has lots of Visa Offices (21 countries at last count). But not all of them have the manpower, or are equipped to handle processing of applications like Permanent Residence.
They instead, form part of the International network that IRCC uses when they need local verification where officers in those visa offices have better knowledge of local laws, customs, and labor markets, etc.
It makes little sense to have all of those offices process PR applications, from a cost standpoint, and some of them are purely a result of diplomatic relations.