I'm not sure how you've went so far with this. Ok think of it like this.
You've got 2 imaginary nations, nation A and nation B. Who both have the exact same coaching level, infrastructure and facilities.
Nation A have a population of 20 million.
Nation B have a population of 5 million.
What would happen is nation A would consistently produce top players at a higher rate than nation B. Simply because they have more footballers.
Nobody is saying that coaching, infrastructure or facilities aren't important. Only that they are joined by population as being another factor.