Streamers get a free bankroll, but are not allowed to cash out the money they earn as far as i know. They are paid extra for it, like in a normal employment. Either by Bitcoin or actually in dollars.
For casinos the advertising value is of course enormous and the player takes much higher risks than if it was actually his money. So it's win-win!
I am not entirely sure if they are not allowed to cash out. I have chatted with one back in the day, in those days crypto gambling was starting barely new, it was 2015 or so, so it was like maybe 1 year or so since the big names started back in the day. He talked about how he was getting the winnings cashed out but not the main money.
So, let's assume that he was given 5000 dollars to gamble, that is what he will pay back or lose to casino, but let's say after 10 hours of streaming which was the agreement, he is up 2000 dollars, then he turns the 5000 back and keeps the 2000 dollars. We also have to remember that these guys keep getting donations and subscriptions as well which they are making money from, they may get some ads as well from gaming companies like alienware or whatever which is not the same industry so they can get those ads as well together along with the gambling, selling mouses or keyboards on amazon affiliate etc etc type of stuff. It's really a great profit maker if you are an entertaining person.