Life is all about experience, and learning from it. Experience gives us certain patterns of recognition. Even if we don't experience something ourselves, through our past experiences we can predict the patterns that will play out.
Overprotected kids have no life experiences to base their patterns on. They have no patterns, except that "everything will be taken care of by someone other than me."
Breaking into life alone, with no experiences, no patterns, no skills, and no ability to survive leaves the kid/adult with little chance. Their entire life everything has been done for them in a vacuum. But life isn't a vacuum; it's a learning experience.
When these kids/adults are finally introduced to the real world, they have some horrible realizations. The last 20+ years that other people have been constantly learning life skills, they have been learning absolutely nothing. They are still kindergartners surrounded by people their own age who are university-level (or higher), experienced, and tested.
Highly over-protected kids just have no chance. Sure, some of them may quickly learn how to live life without being coddled and doted on, but most of them probably just end up leading less than ideal lives.
The people they loved the most (their parents) basically showed them a side of reality that doesn't really exist. It's a type of poison, really. When the kid wakes up in reality, they find that their entire prior existence was a lie, and that has got to be a hard pill to swallow.