When you see sports betting and betting lines promoted all over the internet, ESPN and beyond, do you think it's done just for people who live in or visit Las Vegas and Nevada, the only place where you can place single-game wagers? (Yes, you can also place parlay wagers in Delaware and Canada.)
If you think that's the case, I'm sorry, but you're an idiot. Sports betting is promoted in the US because it's a thing a lot of Americans do, to the tune of some number that is surely more than $100 billion annually, and likely more like $150 billion or more.