
It's because Bundesliga is just a league of B-teams for Bayern Munich, giving young players experience so as to eventually move to Bayern for peanuts or on a free transfer.
Here in Italy, everyone hates Juventus and there are riots when a team like Napoli or Viola sell a top young player to Juventus.
Bundesliga teams can sell globally but they dont mind selling to Bayern. How old or young the players are doesn't make a difference to this behaviour.
I think the difference is more down to scouting practices, footballing culture and attitudes towards the young and things like that.
