Ask yourself this question: is it better to have the skeleton inside or outside the body ?
When the skeleton is outside your body, it shapes a protective shield around your body. The flesh is safe from any external danger but becomes flange and almost liquid.
And when a spike can go through your skeleton shield, the damages are irremediable...
When the skeleton is only a thin, rigid bar inside your body mass, the palpitating flesh is exposed to every aggression! The wounds are multiple and permanent.
But, precisely, this apparent weakness strength the muscle and harden the fiber to resist more. The flesh is evolving.
We have seen humans who had mind-forged "intellectual shields", protecting them from vexation or contrariety. These individuals seem stronger than the average. They commonly say "I don't care", and laugh about everything. But when someone succeeds to really hurt them, the damages are permanent and impossible to heal.
We have also seen humans suffer from any vexation or contrariety, from the slightest touch. But their mind won't close because of that, they remain sensitive to everything and learn from every aggression.