My guess is that it's the younger average population in Africa combined with general higher resistance to sickness from lower sanitation standards in some areas.

But that's just a guess.

Continent Median Age Europe 42 years North America 35 years Oceania 33 years Asia 31 years South America 31 years Africa 18 years

https://www.visualcapitalist.com/mapped-the-median-age-of-ev...