There wasnt many that actually win big when they gamble. Even if they win a huge amount, they probably lose ton they day before. Thats expected to happen because gambling is EV-. Even all those gambling streamer actually admitted that their source of revenue wasnt comes from gambling but from all those streams, merchandise and sponsors. At best, getting break even would be nice for them
Streamers have many other sources of income instead of the money they make on casinos. They are not gods who can change the way the casino works to make the casino lose money for them even if some brainwashed supporters of those streams think so.
They lose money just like any other gambler. But they make a lot by showing off the game and their money on the game and attempt to draw a affiliate crowd there. That is the point of all the marketing there. And also the merch like you said.
Nobody should get the false idea that casino game streamers make a lot of money on the game, its the opposite. I have no idea why this common sense is lacking in some people.
This is just a common sense that the streamers are making money from the stream itself plus perhaps sponsorships too depending upon how much popular a streamer is. They for sure can't beat the casino but only difference is that they show the world that they are successful gamblers and that's their part of trick in order to attract more people to the live streams.