The first of many poker lessons taught in John Dahl's Rounders is that showing any expression is a sure way to lose, unless it's calculated to sucker an opponent into a bad bet. Roulette and craps are prettier than cards, and we don't need to understand rules, just watch the players' expressions. Blackjack requires only simple math, so that's a popular cinematic choice. Just keep the game simple and the sets flashy. Filmmakers love the addictive nature of it, and the bustling environment of a casino or race track is always good for style points.
