Do you have any data for that? It's supposed to follow development and modernization, which seems to still be lagging.
https://www.newsecuritybeat.org/2013/08/demographic-transiti...
Tl;dr Demographers have previously assumed that since Asia followed the same path, the European model of demographic transition was a universal truth. However they underestimated the impact of cultures (in Africa having large families is considered has an assurance as if one of the children succeed the parents will be protected), religion (in Islam even if contraception is legal, one need a valid reason and procreation is heavily encouraged), traditions, genetic differences (I think I read somewhere the age of puberty is a bit lower for people of West African descent) etc.
A documentary about this phenomenon in Niger: https://youtu.be/btMUecYNBc4
Theres a lot of talk about traditions and cultural norms. Do these people realize that the traditions and culture norms in the US, European, and Asia are not only diverse but have also changed over time as well?