I can't think of anything more profitable at the moment. Although I don't know in depth the income from online casinos.
However, to create an online casino, you need a lot of investment and be willing to pay for advertising and interact in a forum like this.
I don't intend to have an online casino, I'm more interested in having an e-commerce.
One of the factors that makes me believe this is that there is already a whole audience ready to play in casinos, as they are generally people who bet out of addiction, to make money (or even when they lose, they play just to pass the time). In addition to the fact that there are costs for servers, employees, online attendants, programmers and other costs, unlike physical casinos where the costs are for installations, maintenance, energy and employees and security guards...
Do they make more profit than digital banks, CEX exchanges, etc.?
I don't know the percentages but usually the bigger the risk the bigger the payout (or loss). So for successful casinos that are running I would think their net gains are pretty sizable. Not sure on margin but with volume comes net wins because in the end so long as you play enough the house always wins. I'm sure there are also tons of other online businesses that are killing it though. Ebay...Amazon, etc those are one in a million but they obviously beat the bank on casinos.