What I think is the other way. Gamblers are saving those funds at the gambling site to save fees from deposits. Transaction fees are hurtful nowadays that's why it was a good thing to do compared to always withdrawing the funds.
As long as the site is reputable, there should be no worries.
That's why if we don't want to get into trouble keeping money in our account, we have to be smart about choosing the casino site so that nothing bad happens later. And if they get a win while playing gambling, they will withdraw the winning money immediately or maybe they will use it again to play gambling.