Because they just don't. Personally, I would say at his age, he's slightly comparable to Ronaldo in how he went to PSV at age 18. I don't think Neymar is quite as good as Ronaldo was at his age; probably one notch below that level, which is still pretty damn good. On the other hand, take Aguero for instance, who also went to Europe at 18. I'd say that comparatively, Neymar is the bigger talent between the two when Kun was his age.
So when you think about it, a player with the potential to be better than Kun, but lesser than Ronaldo, develops about every five years. That may not seem like much. But when you take into account the demand for these players, and subsequently the fact that Neymar has admitted he wouldn't mind joining Juventus, we are talking about far more than five years here. We're talking more like 20 since the majority of starlets will want to play for Real Madrid or Barcelona.
I'd say we have a massive opportunity here, one we should not miss. Even if he isn't better than someone like Aguero in Europe, he should still offer the side with a decade's worth of performances. And that, from an investment point of view, is a smart move.
