Therein lies the rub! Having done it and been exposed to all the wonders and riches of Sportsbooks and Casinos it's very easy to slip over to the other side and look for instant gratification. Once hooked the rest is fairly predictable.
Doesn't apply to everyone of course and just as a balance...
It's not sure it's (just) vulnerable. I'm by no means vulnerable but anyone with a mildly addictive (and worse) character needs to be prepared for what might happen. As Swillis eluded to, most people that fall into this category are in denial; either that they do it to the extent that they do...
I had a real problem with gambling, specifically on line Casino but managed to kick it, found an alternative and didn't go near them for a good 5 years.
Then a mate tells me about matched betting. "It's not gambling" so what the hell. Yes it makes money (not much) and is excruciatingly tedious...
Swillis talks a lot of sense, I'll second him!
I've also been in your position and the truth is you can never say you've overcome it. For me the point came when I knew if I lost more my whole life (work, family) would implode around me. I think I was lucky in finding a gambling outlet I could...