The USA is already one of the best men's teams in the world
No they're not. One of the "best men's teams in the world" would imply that the U.S. has a reasonable chance of winning a World Cup - at this stage they do not.
As for the thread title, soccer is the world's sport because majority of the world plays it.. simply put, it's easily accessible. You can roll up a bunch of socks, use shoes for goal posts and play a rudimentary soccer match. It's a simple game that is not only fun to play, but is also relatively easy to learn. Pass, move, score. It's like basketball in that regard.. you have a ball and the objective of the game is to get said ball into a goal. Easy.
Soccer is a sport that has taken people from living in slums and villages to multi-millionaires living in cities like Milan and Paris. You can compare it to American athletes coming from the ghettos to superstardom, but I'm not talking a college student with an impoverished background making it into the league. We are talking a literal villager making it out of his homeland beyond all expectation, where if not for the opportunities afforded by soccer, they would have little chance of making ends meet.
When was the last time another sport actually stopped a civil war? Mutual combatants in the Ivory Coast called a cease fire because of Didier Drogba and Les Elephants. This is why it's the global game.. it has a profound effect on not just culture, but also politics. The passion is also second to none. Any of you detractors should visit a derby match in Europe to get a firsthand experience for yourselves. We are talking an atmosphere incomparable to anything found in American sports, and you don't need an arena DJ yelling "make some noooooisee" to achieve it. While soccer like any other sport is about revenue generation, it is still not nearly as commercialized as the NFL which is almost like a parody of itself these days.. jets streaming red, white and blue colors, country music stars, pyrotechnics all for an opening game of the season.

. It's becoming borderline embarrassing to watch. I like watching 45 minutes of uninterrupted sports without commercial breaks.. very few other sports can offer that.
For these reasons and many others, it's obvious why soccer is the world's sports. I mean I didn't even need to mention that the super bowl pales in comparison in terms of viewership to the Champions League final let alone the World Cup finals.