My theory is that tribalism is hard coded in our brain, strongly selected for by those bad times in the past, where the ability to turn off emotion and critical thoughts meant you, a generally social creature, could murder your fellow man, to keep your family/in group alive/fed.

I think religion helped reduce tribalism, at a societal level, by making evil/demons/bad acts as the "them" and everyone that went to church on sunday (it was the whole town previously) was the "us". Now, without religion, and the physical/social bringing together it brought, that hardware in our brain still tries to segment a clear "us"/"them", but with much less guidance.