A measure could well be somewhat effective on its own, but then it would require the industry to get creative and work extra hard to still get people hooked, which they will do, but they'd rather not have to do it in the first place.
What's more, opposition to any type of well intended regulation is typical for harmful industries, even if the regulation might be ineffective. They do that on principle, as they don't want the precedent of getting regulated. The mere idea of having regulations for the benefit of society threatens their business models.
Warnings serve to ruin their image in the public eye, which makes opposing further control harder.
As for gambling, there's a simple solution. Ban all advertising of it. If people really need to gamble, they'll find it on their own.
This will dramatically shrink the problem overnight.