Because traditionally in football, if clubs did that, then many of the top defenders anywhere would never have become top defenders, including Kompany who played one game at cb & got chewed up by Peter Crouch & was generaly played as a def mid, Lescott who was prone to involvement in calamity.
Alan Hansen was in and out of the Liverpool team, Gary Pallister was a fucking comedy at Utd. What they all showed, is they have fight & determination to go with their physical mass & occasional excellent performances. As does Mangala.
What you are suggesting, is that if Otamendi struggles after us paying 30 mil for him, we fuck him off next season.