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.