I know it’s a joke, but this is actually the reason. Humans are shit at protecting themselves alone. This goes for weather, predators, accidents. If you’re alone you have a much lower chance of survival.

People with a naturally higher desire to fit in/fear of being shunned or cast out of the safety of the group survived and produced offspring at a higher rate.

It doesn’t serve in quite the same way anymore, you aren’t going to die because of a social faux pas, but your brain will still wake you up at 3am to scream at you that you absolutely will.