Because the poor know best the value of 5$? Even if often they wouldn't afford tipping. But the wealthy won't really know that, 5$ would be nothing for them so

Because the poor know best the value of 5$? Even if often they wouldn't afford tipping. But the wealthy won't really know that, 5$ would be nothing for them so why bother tipping. Unless they actually care about the delivery guy, an attitude which is definitely not a given - some would also think that "poor deserve it for not working hard like us" so again why tipping a "slacker"...