You don't think society has a duty to feed people who would otherwise starve to death? Who do you think should pay to clean up the rotting corpse they leave behind?
But it also has more direct causal motivation: If you don't fight for basic human rights, taxes, plurality etc., you will likely erode the stability of your own life unless you are completely emotionally, economically and technologically independent (which I doubt you are). There is always the possibility of an unexpected economic decline which a rational agent should take into account for maximizing their reward signals.