You have a point. The thing is, with gamblers they prefer to have another person playing in the same table. If the casino put an AI in there instead, it'll be like a noob playing chess on max difficulty - you'd feel very frustrated.
If it is known that an online casino used an AI, that could destroy the players' trust. How sure would they that not all "players" are AI and these are putting an act? After all, it's all virtual, you can't confirm who those are. It'll be like the Truman Show except with AIs.
I agree with the logic you used here - "it'll be like a noob playing chess on max difficulty - you'd feel very frustrated." With AI, the difficulty bar is raised higher, so you have a lot of catching up to do. You will feel disappointed because it is like you have no way of winning the game. So what would be your reaction if you learned that particular casino is using AI? For me, I won't play anymore on that site. Better give at least a good chance of winning to your players. Do not be so greedy!
That's what I imagine a smart casino would do. Just mid-level difficulty on AIs. It's already a game of chance anyway so people would still be losing money. Not to mention they also earn a lot (probably more) from purely chance based games like slots.