There's generally a lot more large wild animals in Africa than anywhere else in the world.
Half a million years ago, there were large animals pretty much everywhere else on Earth too. They very consistently disappeared right around when humans showed up.
Africa has some very fertile regions but except for some like the Nile Valley most of them couldn't really compete well with the Yellow River Valley, the Ganges Valley, etc.
Just for comparison, the population of Africa around 1900 was around 100 million, out of which Egypt had 10 million, Ethiopia 10 million, Nigeria 30 million, and 1-2 other population centers around 10 million each. So 70 out 100 million people were bunched up in maybe 5% of the territory.
In the meantime, just India had about 300 million people in 1900 :-) India within from 1500 to 1900 probably had more humans living in it than Africa from the beginning of mankind to 1900.
Just sheer fertility of regions, navigability of rivers, etc, as a Civilization of EU4 player would put it, meant a "better starting position" for some :-p