The casino operators need to think about the law of large numbers.
If you have an edge, what you want is for a bunch of little guys to bet against you. Your certainty of winning against a million little bets is enormous, if you have an edge. And you do, because those games are made that way.
What you don't want to do is bet a few big bets. You might be smarter than me at guessing the outcome of an election, or a coin toss, or any other event, but if there's only one, you may very well still lose.
If you have a few huge fish, there's almost no point in having the little ones. Your outcome depends on the huge bets.
It's not just casinos who don't know this. I had a friend at a hedge fund complaining to me that he was allowed to risk ~1MM USD/bp on interest rates, but he sat next to a guy who was allowed ~50MM USD/bp. So everyone's bonus depended on the big guy, and everyone else might as well just not be there.
It's possible the casino simply considers the gambler's risk-of-ruin stop to be sufficiently close that they'll mostly hit that stop.