This is true because infants learn based on what they see in the world round them. Therefore, institutionalizing them separates them from the people they learn from. With no one to model their behavior after, the child becomes feral and doesn't develop so when they are finally removed from the institution they are physically not capable of developing the necessary skills due to the fact that their brain has developed to much but hasn't received the nurture it requires in order to complete basic social skills.