Because complexity is the enemy of robustness. A village that lives entirely off its own subsistence farming, or a band that lives off hunting and gathering, c

Because complexity is the enemy of robustness. A village that lives entirely off its own subsistence farming, or a band that lives off hunting and gathering, can keep chugging along in merry ignorance of any catastrophe that does not destroy the local food supply (and if one does, they can pick up and move). A society that depends for daily survival upon massive global flows of food, goods, and information is vulnerable to anything that disrupts those flows.