It's very simple, really: we're susceptible to indoctrination and brainwashing. That's what one of the main objectives of military training - throughout history.

An organised army is the most effective coercive tool that money can buy, so every nation has to have one otherwise it'd be crushed by foreign militarys. And an organised army requires a large group of people who obey orders rather than stopping to debate the wisdom or morality of them charging out to get shot or shoot some other people. An army has to obey orders in a hierarchical fashion to be effective.

That basically sets up an equilibrium where you have a small number of people who can cause the deaths and suffering of millions. To escape it there'd need to be an economic sweet spot where armies couldn't cost effectively coerce small groups of people. But that seems unlikely because the economies of scale to outfit an army favour large groups. We've been lucky enough so far in the modern era that a strong army requires supporting a thriving economy.

Although, you don't know how many species before us caused their own suffering and destruction. Perhaps it's normal and expected. There's a slight chance we'll leave documentation to whatever species comes after us, but they'll probably ignore it, because they know best and it's different that time.

The point is that before the era of modern weapons, many wars were a net positive for the winners. Much of our genome is inherited from successful conquerors, many of whom are long forgotten.

Starting wars has only been unequivocally negative for maybe the last 100 years. Not enough for an evolutionary change to manifest itself.