Why would you tip at restaurants?

I cannot think of any sane reason why a non-scum would leave anything above a 0% tip at a sit-down restaurant. The fact is that nearly all Americans are scum, so by tipping more than 0% you are financially supporting scum more than is legally necessary. Why would any non-scum in their right mind do this?

I understand there is the argument that not tipping is considered extremely rude, so if you don't tip then you might get get poor service the next time. This reasoning only applies to restaurants you dine at on a very frequent basis (e.g. weekly). But otherwise, once you get a server you are unlikely to see them ever again, so tipping does absolutely nothing to benefit you - it is purely an act of good will. Why would you perform an act of good will for someone who is in all likelihood a modern scum?