You live in a delusional alternate reality when it comes to soccer...
The dynamics that created the likes of Messi, Diego, Ronaldinho, Ronaldo or any of the European stars are actually produced by cultures that LIVE and BREATHE football. From the moment that the kids that are born into these nations start walking, they are already dribbling soccer balls and honing their skills... millions of kids are on a similar boat but very few actually have the talent (vision) and work ethic to refine their craft - those few, select kids are the ones that make it through... some kids may even have both of these qualities, but aren't given the opportunity to develop further for various reasons - chief amongst which is lack of financial stability.
Here in the U.S. even if sports such as baseball, football or basketball didn't exist most kids aren't being developed by the same dynamics that are creating these megastar soccer players in those other "footballing-is-life" nations... so the outright conclusion that U.S. athletes would somehow get the country to the front of the line in terms of soccer dominance is not a given. There are many 1st-world nations, with great footballing pedigrees and where "soccer is king" (and even where many of their best athletes are elite soccer players) AND EVEN then, these nations aren't guaranteed dominance over poorer nations. Soccer takes a certain kind of skill, and that type of skill is usually produced by a hard life - and the type of perseverance that such cir stances engender. Most soccer players in the U.S., however, actually come from affluent families that have the time and resources to register their kids into programs such as the ODP (Olympic Development Program) or other similar development programs. Such programs can only do so much.
Since the developmental dynamics are completely different... so too are the results.