When a person allows greed to take over their thinking, it becomes very difficult to convince them they are on the wrong, since once they obtain a huge win, they cannot help to think that if they were to follow your advice then they will deny themselves of all of the potential profits they could get if they kept gambling, so they ignore your advice thinking they know better, and it is only once they lose it all they realize you were right, but at that moment it does not really matter that much anymore.
This attitude is seen, not only in gambling alone but also in other areas of life. People tend to discard valuable advice or constructive criticism simply because they see that it’s working for others, and maybe they’ve tried it a couple times and it maybe worked a couple of times for them too, so regardless of the potential consequences of that action or the advice you give to them, they’ll always feel skeptical about taking the advice because they believe they’re doing the right thing, we’ll not until things go bazaar and maybe get out of control and then they’ll realize that they’ve been wrong after all.