my social theory is simple: economics.  
What can be cheaper than kicking a ball( any any object approximating a ball) around any opening area one can find. Notice how expensive other sports can be like basket ball(court/hoops needed) football: pads/helmets/shows & goal posts/field etc. Baseball: gear/ shows etc... since thats all they play since they can walk. 
You know the saying: practice makes perfect.
 
That's why  Germany ,Spain, Netherland , Italy , England .............etc . are some of the best team in the world 
 and El Salvador,Guatemala, Sudan ,Ethiopia  , Haiti ........................
 ........................
 .........etc . are some of the worst  
