Gates has talked about what has motivated him to donate the way he has, its no mystery. He says he was gobsmacked by how little money was devoted to the health problems experienced by poor countries and that comparatively small investments could have enormous impact.
Also, I think you are underestimating how terrible malaria is
> Globally, the World Health Organization estimates that in 2020, 241 million clinical cases of malaria occurred, and 627,000 people died of malaria, most of them children in Africa. Because malaria causes so much illness and death, the disease is a great drain on many national economies. Since many countries with malaria are already among the poorer nations, the disease maintains a vicious cycle of disease and poverty.