@Advocate,
You are over-simplifying the comparison. In order to compare the 2 countries sports environments, and compare apples with apples, you should not compare the population of countries...but actual soccer-playing populations. Also the length of time the sport has had to mature with international competition.
Considering the length of time US has taken soccer seriously at an International level and has had a strong coach (like Klinsmann), it hasn't been that long. US is now making good progress, but should be considered in it's teen-age years.
In US, soccer competes for a pool of possible talented athletes who are diverted elsewhere. Sports to which they might gravitate is enormous. In Belgium, an athlete who is looking to the top tier of competition and money, there is cycling and soccer. Not much else to my knowledge.
For competition in sports for talented athletes, US has (gridiron) football, baseball, basketball, golf, tennis, ice hockey, cycling ..what am I leaving out? At the early developmental stage of sports youth, Soccer is played in grammar and junior high..and somewhat in high school. Then it trails off. It's been hard to keep moving the potentially talented athletes along and retain them in soccer.