Before someone even starts to conceive that idea of becoming rich through gambling, they should ask their selves if there are many gamblers that has became rich through gambling and that can give them a rethink that they should gamble responsibly and only take calculated risk.
People often act without thinking, especially when the crowd are doing the same thing, they just assume it's right and there's no need to really think about it. Some are pushed by desperation, but just like you said, if most gamblers can only sit and ask themselves this important question, how many rich main in the world got their wealth via gambling? I believe this is meant to at least give them a clue, even if they were to argue that the fact that there's none yet doesn't mean there can't be.