Gambling portrayed negatively because of the main reason gambling can have a negative effect to the person involved. And also I think gambling can lead a person to be addicted,greedy and some will commit crimes just to get a money to feed their addiction.
It is not like Can have no, it must have. We are certain and assured about this thing. All those expert gamblers of their times, once they get to know what they have been doing in their lives since times, they curse them selves. They start yelling out at them and regretting but of what use now? Their advices never goes in favor of gambling, not a single one. Keep yourself away from this harmful game.