It's not necessarily a problem that's specific to soccer. In baseball, the Dominican Republic can field a national team that's at least competitive with, if not better than, the USA. Same in hockey, where Canada is equal to/stronger than the USA despite having a tenth of the population.
There may be some correlation between the population of a country and how strong that country is in any given sport, but there are so many other factors that go into it that are much more significant than population. Culture is probably the biggest factor; how many of the 24 million (or whatever the number is) who play soccer in this country are really passionate about the sport, vs. how many play because their parents signed them up or because their friends play? How many are kicking a ball as soon as they can walk, and how many watch games religiously? How many soccer players in the US view the sport as a potential career, where they can make it as a professional? If you were to compare Croatia to the USA in these terms, you see why they're playing in the WC final and we didn't qualify. I think that we're making slow progress in becoming more of a soccer country; comparing the soccer "culture" in the USA in 2018 to when I was a teenager (1970s/1980s) is like night and day. But it is slow progress.
There are other issues that explain why a small country like Croatia can crank out so many great players and we can't. As others have said, the pay to play system and subpar scouting/talent identification are also factors.
Having said all of that, I'll add this: USA wins the World Cup, in 2026.