Well that's the point, actually the reason is very simple why borrowing money to gamble is really an action that is strictly prohibited, the reason is as you said that there is no guarantee or certainty whatsoever that we will win at the end of the game where of course the action is done because of the encouragement of hope and belief that they will be able to get a temporary victory when it turns out the result is actually the opposite then that's where disappointment, emotion and regret will arise which means that borrowing money is clearly an excessive action which in the end in most cases only leads a gambler to emotions and various aggressive actions that will be more dangerous to themselves and that's the reason why borrowing money is better avoided.
Well from my point of view, borrowing might not be strictly prohibited but it is just not compulsory and it is a personal choice that is still not advisable to do. Anyone who goes to borrow to gamble should know the uncertainty and risk behind such actions and should take caution because gambling is very risky and as such anything can happen. This act alone could lead to addiction or it is a clear sign that addiction has set in otherwise why would a gambler go to the extent of taking a loan if not for addiction when we all know gambling is for fun and we can gamble with what we can afford to lose.
Hmm yes actually it is not a problem to borrow money because after all loans are an option that is indeed made to be used as an alternative to overcome emergency needs, and it is a fact that gambling is a desire and not a need because there is no obligation or coercion, but yes it is not a problem to borrow money to gamble as long as they know the risks as you said, meaning whatever will happen at the end of the game then they must be able to accept it with an open heart especially defeat.
Another thing yes I quite agree with you that most likely they have been in a cycle of addiction, or have been trapped in it because of seeing their actions that force the situation in the sense of borrowing money to gamble and of course that is an action that leads to excessive aspects, so I think that the idea of borrowing money to gamble will never be justified, because rationally when a gambler only considers gambling as entertainment then they will not force the situation just to gamble.