What I had in mind is the simple thrill of winning against other people, and being able to do it from the comfort of your computer/phone, without having to actually deal with people face-to-face.

In this sense, gambling is just about rising the stakes to get that emotional hit when just fake points don't do it for you anymore.

I'm assuming that most online gambling is like poker where there is a degree, or at least an illusion, of skill and agency. Having some randomness involved is also quite appealing because you can keep hoping to get lucky and can win now and then even if you are not great at the game, it can still be fun to win even if you didn't really earn it. But I doubt many teenagers just keep pressing a button like in roulette or slots.

Well there is loot-boxes of course, but that's more engaging since the rewards are tied to an actual game, there's the chance to show-off your winnings in the case of cosmetics, the collectable nature of the rewards, and sometimes the possibility to trade them for other items or real money.

EDIT: There's also sports gambling which is massive, although not sure if there is any overlap with these people that flow from gaming.