I think people tend to be bad at difficult tasks they are not specifically interested in, and we're all better served if people tend to be able to work on what they're interested in. I suspect that's going to end up better than if we somehow create a list of world problems ranked from the most to the least important and get the entire world to work from the top of the list down. (Never mind the difficulty of creating such a list.)
There are charities that focus on reducing human hunger. There are also charities that focus on reducing child hunger. Why should the latter exist, since it's a subset of the former? Isn't everyone working on the child hunger charities ignoring the problems of adult hunger? Of course not. There are just some people who, for whatever reason, are specifically interested and motivated in fighting child hunger, and I strongly suspect that overall outcomes are better when those people have the opportunity to focus their work on child hunger.
There are charities that focus on reducing human hunger. There are also charities that focus on reducing child hunger. Why should the latter exist, since it's a subset of the former? Isn't everyone working on the child hunger charities ignoring the problems of adult hunger? Of course not. There are just some people who, for whatever reason, are specifically interested and motivated in fighting child hunger, and I strongly suspect that overall outcomes are better when those people have the opportunity to focus their work on child hunger.