They keep a percentage of the pot, and hosting tournaments also brings in a lot of money, both from the participation fees and from the event organiser. But poker is not really there to make money for them. People coming into casinos rarely just play one game. Poker brings people in because it may be interesting to watch, especially high stakes games, but most people going into casinos are hesitant to play poker there because they’re expecting that the players there are better than average.
In addition to the other answers. Sometimes they’ll bring “house players” to the table. They’re people paid to play with the casino’s money. Usually they’ll be put in to keep the tables full. But other times to keep some of that money in house. How much the player get to keep of whatever they win will vary place to place.
In a cash game (a game where people just play poker), the casino takes a rake (a percentage with a certain cap) from each pot that is wone. In other cases, like in card rooms, you might get charged by the hour. Either way, you’re basically paying a “rent” to have a table, and in the case of a casino, a dealer, plus what is presumably a game that is safe (from cheating, from players trying to rob you afterward, etc.).
What hasn’t been mentioned here is the real reason that casinos have poker rooms. They have poker rooms to get players in the door. A lot of poker players will also play pit games on their way in or out of the casino. Even if they don’t do this, their wife will sit at the slot machines and play for hours while they play poker. It’s all about getting customers in the door.
It depends on the Casino, when I was a college student I used to be a drink server at a Casino. Video poker was open to everyone and anyone on the slot floor, but the actual poker tables were in the High Roller lounge where only people with casino memberships and VIP who spend and win a lot of money are allowed to play. There was usually a % cut the casino got from the pot.
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