United States are known for gambling, the highest amount of losses comes from the US and it keeps on increasing on a daily basis. Everyone knows that Las Vegas is the center of every gambling activity. This country has more addicts, and I will it's this way because of how easy it is to get money there. Even though people struggle it's not like a third world country that I live where getting a job is a problem.
The fact is USD has a pretty decent value in the currency market and as a result of the fact that it's the base currency in the US, their volume will definitely turn out as one of the largest. Come to think of it , statistics like this are not drawn based on head count or something related rather it's mostly based on total volume.
Aside from that the frequency of gambling in a country also depends on the intensity of the labour sector of that country. It isn't a directly dependent factor but it also contributes indirectly too.