I doubt it is more about the profit in gambling, some people I know just gamble to spend their spare time but other gamblers usually play to make money. It depends on the person's perspective but the same end is expected for both category people. My opinion is the rich gamblers are less than other types of gamblers.
There are gamblers who are into pleasure than making money and the reason why rich is lower is simply because the rate of rich people are lower too and most of the population are came from the middle and lower class. Every gambler have their own reason to gamble, personally i don’t gamble to make money because I know that making profit in gambling is not that easy, so better not to take it seriously.