What is the reason for playing any computer game? To get a dopamine fix by reaching some virtual goal. People pay small (at least at first) amounts of money for access to the game or in case of free-to-play games for a better chance of getting to the next goal. If someone gets a bit of excitement every time they win a virtual jackpot, they might think a $1 is an acceptable price to pay for it. And then it just spirals out of control.