The comments here are wild. Uh the answer is football/basketball/baseball. We send our best athletes into football, basketball, and baseball. They don't play soccer. I would argue its more shocking that we are as good we are considering the talent pool.
This comment is wild :) In Europe we send our best athletes into athletics, not sport. Athletics is track and field, which reduces to speed, strength and endurance. Sport - football, cricket, tennis - is about technical mastery first, and only secondarily the speed strength and endurance to support it. Lionel Messi and Sachin Tendulkar would both have been deselected by any athletic selection procedure. The emphasis on athleticism over technical mastery is what makes American sport boring.
The root cause is that no one watches soccer so there’s no money in it. It’s not like there’s a limit of 3 sports we could send athletes to, but since no one watches soccer here there’s no money for fields, equipment, training, coaching, athletic science, etc.
It's slightly more than that. These sports (and let's include hockey, since it's played widely intentionally too) are all organized the same way, meaning the players are largely trained the same way leading up to college and also in the pros. There's a clear pipeline to the basketball and hockey national teams. The best league is the one closest to home. The uniform pipeline and playing close to home makes training for, coaching and playing with the national team that much easier, which makes the team that much better.
American men gotta find their own way to become a world class soccer player. There's no pipeline like there is for the big 4. It's harder for teammates to gel when some went through the ranks in Germany, some in England, some in Italy and have only a few weeks with a new coach to buy into the system.
America's biggest rivals also aren't very good. So while the best European and South American teams constantly have to play each other and fight for survival, the US has to play middling teams like Mexico and Canada and tiny Central American and island nations.
I don’t think this is fully correct. Athletic skill is not universal across sports. A athlete that can be trained to be a star football player may not be able to be trained to be a star baseball player.