I disagree - most wingers these days played most of their junior footy at centre/fullback. I can understand this, as the physical step up from NYC to NRL level is huge, and it takes time to bulk up.
The result is that we have very few specialist wingers these days. Sing was the best I've ever seen. Ever since he left the Cowboys have struggled to either score points or stop them - we can't seem to find a winger who can do both at the same time.
Also, John Williams was IMO the best Cowboys player in 2008 - and he's a winger. It's much harder for a winger to stand out because they don't touch the ball anywhere near as much as the other positions, so this is no small feat.