Only reason I can think of is that they want to eliminate cheating.
If someone finds an exploit he could keep withdrawing as much as he'd want.
When you own and manage a Casino you must take the risk for the security of the casino and games... not the players.
If you dont have 50 BTC to pay all winners in less than 2 hours, you should even be in the business lol
I only play in casino who got fund to pay .. that the goal of gambling ... winning isn't ?
I advise you to check the terms of the casinos before playing anywhere
I agree that it can take few hours to move funds from cold to hot wallet but I can't understand casinos paying less than $2000 a day