Because we are animals and it’s a way to indicate to someone and others that we are hurt. I’d say an evolutionary thing but more just natural biology. Which is evolution, but even back to aquatics, there was some sort of vocalization I’m sure. So it’s always been there
also the exhaling and vocalization of a yell helps ease pain. Mythbusters did an episode on this, though it was more on swearing but same difference
So because all non-single cell organisms that are sentient/sapient (humans and non human animals) benefit from vocalization, they do. and sometimes on a level we can’t hear
obviously there’s probably exceptions.
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