Why don't passenger vehicles have removable rear quarter panels?

The way they mount parts of the body usually have to do with the nature of impact common to its location. The reason we can have modular front fenders is because they are part of crumple zones. The thing about crumple zones though is that they only spread the energy out, they don't absorb it. The firewall is what absorbs it. Unless we install firewalls in the rear there isn't a safe way to have rear crumple zones. Instead of adding all that extra weight and wasting material they just make a more solid fender in the first place, and it seems to work considering that most parents trust the backseat to keep their kids safe.