I am asking this because I do have some short of an answer for videogames and for massive multiplayer games and even for social network oriented games. Videogames require a great design, good difficulty grading, an story that engages the user, massive multiplayer and network oriented are a lot about the interactions and how varied and fun these are. However, for casino games, it would seem that these factors are not really the most important ones. You can see games with very basic graphics and design, simple in the options for the player and not that engaging yet they do "hook" people in them.
I believe games are already out there so I do not know if there could be a correct answer aside from "many people do play it" and nothing more. Surely you pick your own poison but they are out there, there is nothing new, you give it a try for each of them and play them a bit and then you find the one you like so much and continue to gamble with that.
Video games also do not require a great design or anything like that, I have seen a ton of text based games that got a ton of attention as well, as long as it is "good" people will be hooked, it is not just about one aspect of it being good it is about all of it feeling fun for people. Look at stardew valley it has 8bit graphics that is probably 20+ years old by now and it sold insane amount of copies because it is a great and fun game.
There isn't anything that is so challenging for it neither, it is quite simple, probably the simplest game I have seen while having very old design, and yet it is still fun. Same goes for gambling, as long as it is fun nobody cares about anything else.