I think what you’re observing is the interplay between two variables with opposing correlation with respect to wealth:

1. Having empathy
2. Ability to display empathy

Poorer people might have more empathy, but their ability to show it is inhibited because of lack of resources (time/energy/material). Wealthier people may have all the means to display empathy, but they’re less incentivized to do so. At some point in the middle, you get a sweet spot where there’s both sufficient desire and ability to do good.