If the hull of the plane was made of super sturdy material, the passengers inside are not. The sudden deceleration would have all the passengers flung towards the front at fast speeds.
It’s the same reason cars are built less sturdy than vintage cars. They are designed to crumble so the deceleration isn’t as fast.
Force=mass*acceleration
You can’t decrease the mass, but if you decrease the negative acceleration (deceleration), you can reduce the force of impact
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