Well, nothing is so surprising about the USA leading gambling in every category. As we can all see, sportsbook, odds, spreads, and other sports betting related terms are what the sports casters talks about every single game. Even the commercials, casinos are always present. They are even a huge sponsors in mini games happening during timeouts and halftime.
However, I was expecting some countries in Europe would dominate the top 5. Looks like they're the kind of luxurious gamblers or maybe I'm just stereotyping here lol.