A lot of MMORPG actually have gambling elements involved, I think that it's not simply the thrill of victory that is the rationale behind gambling, but instead the thrill of potentially winning a life-changing amount of money, something which can't be done on online games through simple gambling.
I do not know if you have ever tried, but have you tried to play some of the games in a casino without real money? Because if you do you will realize that the games by themselves are not very fun, what adds the thrill and excitement is the possibility of earning and losing money, on the other hand video games are designed with the intention of being fun from the very beginning, so it is very obvious to me that the nature of the games are very different.
This explains why those that gamble are not satisfied by just playing video games, the concepts in which they are based are completely different so a person that gambles for fun is not going to necessarily like video games as a substitute.
It would still depend. Others are just gambling so they have something to be addicted to and now that their money is draining because of that bad habit, they have to find a substitute which isn't as expensive as gambling. In online games, you can find a community. That way, you're going to have a sense of belonging while being hooked on something. Before, I used to like gaming, then I didn't, and now I do. It's all about finding what games are for you and what aren't. Just because a game is trendy, doesn't mean you're going to enjoy it as well. And sometimes a game might have few players but still be enjoyable for you.
As kids, we used to substitute punishment for reward. You get to punish your opponents when you win or your opponents get to punish you when you lose. It's still the same reward mechanism going on in your brain which makes gambling fun. Without it, you're pretty much just practicing for whatever game you're playing.