It's just that those regions are warm and humid, and this creates great living conditions for mosquitoes.

To make things worse, many countries in these regions are poor, so they can't afford treatment or prevention (pesticides to kill mosquitoes, draining swamps, etc.).

Rich countries in the same regions most likely don't have big problems with malaria (or if malaria is still a problem, it's probably a minor problem).

It's just "bad" geography (from this point of view) and poverty.

If you see someone drowning and refuse to throw them the life ring because you might damage your newly painted nails, you're not technically killing them on purpose, but you're still killing them through your inaction