Do you think that's it? By that logic, I could make a game that's ridiculously easy and become a millionaire like that. People will beat it, no effort, success.

I think there's got to be a perceived effort and learning curve involved.

That can get people hooked initially.

The tricky part is to get people coming back. That involves increasing the difficulty (but not too much), so the user feels a sense of accomplishment.

Throw in elements of randomness and, voila, you have an addictive game.

There are studies that go into more depth than this, but that's the basic outline.