Question

Why are car seat belts designed to stretch somewhat in a collision

Answer 1

Because it exert equal and opposite force on the body when when the body tend to move in forward direction.
Hope this helps you friend
Thanks ✌️✌️

Answer 2

Because when seat belt is being hard or tight then its effectively affect on our stomach, and stomach can't able to absorb or take hard seat belt ..
And suddenly our stomach has been started paining...