It's perfectly rational to not want to save the world if the world isn't in danger, or even if the ROI of dealing with threats to the world as a whole is less than dealing with more local issues. Knowing that humanity will continue is cold comfort if you spent your last dime to accomplish that and don't know where the next meal is going to come from.
Given that we're all part of it, the question should be "why are you not always trying anything to save the world?"