In a given jurisdiction (State/Province/County) there will be rules about the payout percentage. Anywhere from 90 to 98%. So lets assume 95% percent payout. In the long run, you give me a dollar, I give you back $0.95. Most of the time its zero, sometimes its a bit more than you put in to spin and once in a blue moon its huge. But, on average, you give me a dollar, I give you $0.95.
This is where biology / psychology comes into play. All the games from all the manufactures pay the same (in the long run) in a given state (its the law). The trick is to find the magic reward frequency (think Skinner box from Psych 1000). Occasionally a huge one with not much in between? Lots of little wins and hardly any super huge jackpots? Most of the reward frequencies don't work. The person can escape thinking the game is not fun. There are a few rare evil/magic frequencies that just trap the poor person.
Here is where it gets super evil. When one manufacturer's new game starts to do well in market, other manufacturers will do research (study the content of the reels (the spinning things) from a video capture) and reverse engineer the game's math/reward frequency.
So the same way that AI RNN is a exploring a multidimensional problem space and trying to jump out of local optima to find the true optimal solution. The gambling industry is exploring the dimensions of the mind trying to find the optimal Skinner box to take all your money. They are exploiting a bug in our brains design. We evolved to run around the Savannah and throw things with weird accuracy. We just are not built to defend against it. They should be banned.