It never was. People were waging wars long before money existed. In fact, early conflicts were smaller in scale but far more brutal - mass graves across Europe are proof. Before tribes started trading, there was no concept of negotiation. Other tribes’ resources - including women - were just there to be taken. They’d invade, kill even the kids, spare only women capable of bearing children. This was happening hundreds, maybe thousands of years before barter showed up. Money and trade actually improved life and reduced the brutality of conflicts immensely, but they didn’t rooted out the core issue. Wars were never about money. They were about scarcity of resources and alternative uses of these resources. As long as scarcity and alternative uses exist in human society, wars will always follow.