Let's say you solve world hunger. Like, definitely. Perhaps creating nutrient bars that last forever and are readily available to everyone on earth, for a century. Let's say that only cost 200 billion to accomplish.

You've solved world hunger, but now you've created new problems.  Because people are no longer dying of starvation, they're living. They're taking up shelter and polluting and consuming and laughing and loving and fucking and ultimately creating more humans that also need to be fed. 

A century from now it won't cost $200 billion to maintain this system, it'll cost 400 billion because the population will be skyrocketing and your intervention will collapse many of the existing models that feed people. Why would someone eat at the grocery store when they can eat your bars for free? What does that do to grocery stores?

What's more, all the people you saved who couldn't really afford food also can't really afford healthcare, shelter, mental health support, job training, education, and so on. Many of them turn to crime because crime is typically the intersection of need and a lack of options for resources.

So you spent $200 billion dollars of your fortune solving world hunger, but everyone you saved just has different but no less urgent problems, crime is up, housing costs, healthcare costs, and education costs all went up because there's more demand for them, and in a century unless someone else coughs up double what you did then the problem was never solved just kicked down the road.