Maybe the better way to represent the disparity is with remainer average reward per post divided by leaver average reward per post rather than the other way around. In that case the correlation changes from -0.182 to +0.182.

I think the most likely answer is that rewards are not actually a major factor in people leaving. There's a wide range of other possibilities. Anecdotally, among my family the reason almost all of them say they don't use it is because it's too hard to use. I think we'd be unlikely to find that answer with a SQL query though...