Because they are not 'proven' in the Italian way?
Sahin and Neymar are going to be great, if not amazing.
young players like this, any player actually, would need the right environment in which to develop and express themselves properly. It remains to be seen if Real Madrid with all the high expectations and pressure to always win, resulting from monstrous investments, and lack of patience with under-performing youngsters would turn out to be the right place for players like Neymar, Sahin, etc to properly develop their game.
I'd say their chances for failure at Real are higher than those at most other big European teams but, I guess, time will tell.
Imagine Milan selling their most important player, the key factor to winning their title just one season after he bloomed.
if a player really wants out of a team, there is nothing that team can do but maximize their profit from the sale ... it's in their best interest, in the end of the day. Luckily for Milan, this is not the case with TS - I am sure he won't cry if he is sold to Barca but he is definitely not pushing for the transfer himself.
This being said, right now Barca cannot afford TS with all the other targets they have in mind - last summer, when TS was still finding his way at Milan, the club refused an offer of 35+ mil for him ... imagine what type of money would take to get him out of Milan now when he has become possibly their most valuable player and one of the top defenders in the world, in general.