At the younger age groups, there is a lot in what you say. Its a fact that some smaller clubs will actively hide their best youngsters from International recognition, for fear of them getting noticed and poached - certainly below scholarship level.
Once you get up to U21 though players are...
Well, that is clearly true, but remember that the Albion % of call-ups that prompted the starting of this thread, is also doubled by the inclusions of Walton, signed for a significant sum at the age of 17, and Solly March (re)signed from Lewes at a similar age.