It’s quite true that Africa contains a lot of countries, but it is only marginally higher than Europe (54 vs. 44).
Your response brings out two excellent points: 1) More travel to Africa would certainly help — because once you have visited a place, you tend to understand the place a lot better, and it becomes a very distinct category in your mind. 2) Prioritization, picking out a few key countries to learn about, is also very helpful.
If I were preparing a course on sub-Saharan Africa, and if I felt that I had to concentrate on only 5 countries, then perhaps I would choose Nigeria, South Africa, Congo, Senegal, and either Kenya or Tanzania. But if I could expand the list to a dozen countries, then perhaps I would also include Ghana, Ethiopia, Rwanda, Cote D’Ivoire, Zimbabwe, either Angola or Mozambique, and either Liberia or Sierra Leone. On the other hand, if were planning an upcoming visit to Africa, then I might be tempted to include places like Namibia, Botswana, Madagascar, and Uganda.