First and the main one is the affiliate/referal programas, this way by sharing your affiliate link you will get some incom from the users bets, isn't easy to get good affiliate (users who bet a lot) but with one high roller you could make a descent amount.
The second one is the VIP programas, like the one that stake offers, you need to bet to grow your account, but once you have a good VIP level you will get a weekly and monthly prizes, It will be better if you bet that month but even without betting you will get something each week/month.
Those methods can be called ways of earning money from casinos and not gambling because when we say gambling, it gives the expression that we are actually making bets for the money that we are getting, but that's not the case here. Besides, these methods can be used in any platform to earn money and not only in gambling. The thing is that you might get more money using these methods at a casino since people gamble a lot, at least some people do.
When we talk about affiliate programs and earnings, it's an extremely difficult thing to do because getting people who would keep making purchases or deposits is a hard thing to do, as you said. It was easier in the past because people didn't know much, but these days, every other person knows about everything available online.
If you ask someone to register under you, they will say they are already registered. This is why it becomes very difficult to earn money using these methods.