What are you talking about?
Just because you SEE guys at the highest level (let's say BPL) playing predominantly at one position doesn't mean they haven't played multiple positions throughout their "developmental" careers. Or even now. Time, score, opposition, injuries, red cards,etc., demand in-game versatility. For example ...
Man U. Since August 2011, we've seen Carrick play MF AND center half in the same game. Smalling has played center half, fullback and (even briefly) up top. Valencia has played MF and fullback on BOTH sides. Jones has played MF, fullback and center half. Even Rooney plays variations of his usual "strikers' role, depending on time, score, formation and partners. I could go on. Heck, the whole Dutch system of total football is built on player versatility.
Specialization at an early age stifles player development. Pigeonholing a 14-year-old is the height of stupidity.
That said ... I understand why others see it differently. If the priority is winning, or accumulating meaningless statistics, by all means, create a bunch of highly specialized automatons who can't make in-game adjustments.
Come to think of it, sounds a tad like the USMNT.