Gambling isn’t mainly for the rich but seeing as you do need some money in order to play, one could see why gambling is said to be for the rich in the society. I think it’s for anyone with some money to spare and willing to take some risks.
If you’re rich and get into gambling, the obvious advantage is there is money to comfortably bet and such individuals wouldn’t look to gambling as a source of making an income.
Generally, although skills are needed to try to identify and predict games that could be favorable, you literally can’t do anything without having some money in your pocket. You can have all the skills but without any money, you’re pretty much a spectator.
In general, many rich people gamble just for fun not victory, because they have a lot of money so they don't mind the defeat they get, and they don't make the defeat they get a big loss so they don't pursue victory or loss, but for poor people it's the opposite.