Neanderthals had larger brains than we do. If intelligence was the determining factor in out-competing rivals, we would expect all species to evolve greater intelligence at the cost of other adaptations. This is clearly not the case: only one species invented the gun, all others went with sharper teeth. Even now there are more sharks than dolphins, more reptiles than mammals, more insect than reptiles.

But in humans, the very idea that "we" fight to gain "their" requires creating an "us vs them" dichotomy. But all humans are the same species, and persons on national borders are invariably more closely related than on opposite sides of the same country.

Essentially every border is an accident of history: Romans building a wall, Charlemagne having so many heirs, monarchs marring one person vs another, religious boundaries becoming national ones, the treaty of Versailles drawing straight lines through deserts.

To convince people that a particular "us vs them" dichotomy is real requires the listener to have a certain kind of thinking processes. Especially to convince them to kill and die for it.

And because every population falls for this, any individual population that smartens up and notices it's absurd will likely be wiped out by those who still have the "us vs them" delusion. Thus the species is kept in a sub-optimal state.