Video games are, by default, designed to be addicting to keep users coming back for more and for longer periods of time. Game developers pump out engaging new material as often as possible to keep their audiences primed and interested. Most games are made to be hard enough to be intriguing and rewarding, yet not so difficult that they drive people away out of frustration.
This, coupled with micro-rewards like in-game achievements and daily tasks, ensures that players feel a sense of satisfaction every time they log in and causes them to be more likely to return. Video games provide a sense of satisfaction and accomplishment that triggers the release of dopamine by activating the reward center in the brain.
Whenever a player completes a task, levels up, or progresses the storyline, the brain releases more of this feel-good chemical, keeping them interested and making them feel happy.
Because playing and winning games feels so good, the individual feels compelled to continue doing it, sometimes for a long period of time or at the expense of other obligations.