It comes down to values. Our values have been shaped by stories that on average yield a high reward for a lot of people and they are based on aesthetics, empathy, social stability, food, peace, absence of pain and chaos and so on.

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.

Because I'd rather live in a world where a few pennies out of my pocket results in everyone living a decent life, sans street corpses. Seriously, even if you're an extreme individualist, is there NO price you would pay to not have dead bodies strewn about the streets?