I guess one of the reason is that , the rich don't actually take gambling as a big deal or a means to earn , they just literally do it for fun .and they don't actually care if they will or lose , what they care about is the fun they got from it. And also the rich don't actually use small cash to bet rather they prefer huge amount of money and it by any chances they hit the jackpot they will endup making tones if profits. While am average Man will always mind the way he or she use their hard earn to bet , like most people don't literally bet when they don't have extra funds to spare inorder to coverup some expenses .
You seems to have captured my perspective of this matter because I realised that gambling is highly connected to the human psychology such that the moment you get too attached to the money, it will become very difficult to make progress. Perhaps, this may be reason many gamblers have adopted the concept of gambling for fun and not for the money, the psychology behind this being that they have money to sort out their lives and gambling is not just for the money. This can work and may be the reason the rich win more than the poor, they don't have their mind fixed at the money and can cope with losses without being depressed.