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.
I agree with that, gambling is a game wherein we could gain profits(if we are winning) or we could lose the money that we earned by working hard(if we are losing), however since most of the players are losing every time they are playing gambling, that is why they can't help but to give negative feedback about it so the other would be discourage on playing it which is a good thing because gambling is a very dangerous game.