Because losing is inevitable on this game, and you can't force the gambler to pay if he lost all the money.
This is why a gamblers can easily be digging his grave of debts after debts from everyone.
Lending money to a gambler could be very risky because there's no certainty of payment except the gambler turns out lucky almost immediately but then at that it also happens that greed could set in and they would wan to further grow the money and may end up loosing it again to the casino and most likely come to beg for an extension in the payment time so when lending a gambler just be aware of the risk involved and the gambler my further add to their Dept in the bid to recover.
Honestly, I don't care whether you are financing a professional gambler or not, but gambling is all about winning and losing, if there are wins it means there are losses, and obviously the casino's goal is to profit from losing gamblers, so with that alone you can be sure what your fate will be. Instead of getting a win but the opposite happens, defeat dominates. On the other hand, even if it's just a loan, I still think there are doubts, I don't believe the person you lend it to will be able to repay it if they have an active gambling habit.