Because it's not a healthy industry. It's a vice and not a hobby that could make you more productive or focused in things that you do. You could easily get sucked in type of situation that feels like hell where getting out is another miracle.
Why wont be anything that is harmful, as you have also mentioned, be portrayed negatively or prohibited? Gambling has the power to ruin anyones life. This world has only few gamblers that really became rich by this game. I have seen people committing suicide because of losing all capital. Anyway, a sensible person who loves his life wont go for such an option which can turn his life into a living hell.