The sheer size of the USA means that it's bound to have some good players even if it isn't one of the main sports. It's by far the biggest country in the competition; much bigger than Brazil, four times the size of Germany and six times the size of England.
It's also worth bearing in mind that passion for following the game doesn't always imply passion for playing the game, and England is an example of this. The USA is an example of the opposite, since participation in the sport is surprisingly high. In England, the idea actually playing football at any high level is rarely taken seriously in middle or upper-working class households (a cause and result of the fact that English footballers tend to be ill-educated louts and that becoming a professional usually means dropping out of school at 16 and getting no education), meaning that it's really a sport for an increasingly small proportion of the population. England is actually very close to the US in terms of participation in football, and it's nowhere near as much of a footballing nation as most Western European and South American countries.
