Sometimes they are mafias running the business or spies trying to bring them down (James Bond in Casino Royale, anyone?)! There’s tension all around, till the protagonist walks away with his prize (either cash or his life). They let you live vicariously through the characters on screen. And, movies about casinos or based in casinos give a riveting glimpse of the glitzy life where stakes are always high.
There’s something fascinating about the roll of the dice, the turn of the wheel of fortune (big six wheels in casino language or testing one’s luck at roulette… the casino gets even the non-interested, intrigued.