Milan isnt a developping team, they are supposed to be a title challenger.
Because they have a bad few seasons now, doesnt mean they became Sassulo.
If Mexes, Essien, Abbiati are at a higher level then the youth you mention, and they most certainly are, then its logical at a supposed topclub that they are picked ahead
This is why you dont fill your bench with youth, you loan them out instead so they get games, because top clubs are about winning, not at beeing a Nanny