One problem may also be this - the links between African countries is weak. For example, if I have an office in Singapore, I am basically covering all of South East Asian very effectively. I can move people and resources across SE Asia from this office.

If you have an office in Lagos, your reach is limited to Nigeria, Ghana and a few other West African countries. The link between Nigeria and Cameroun is weak, and Nigeria to Congo is a difficult journey. To make a trip from Cameroun to Angola, it is probably cheaper to first fly to Portugal and then back.

Africa is a huge place and there are regional circles. So if you bother to make an office in a particular country, you are not covering the whole of Africa (which, combined, probably comprise a good chunk of customers), you are covering just the regional zone that has relationships with that country, and these are probably a much smaller group of people, making it not worth it to have such an office.