We still have war because we've not yet matured as a species. Greed, hatred, fear, power lust all cause wars. What is said out loud is merely justification for those emotions. And we still have wars because people (nations) know that to allow criminals to get away with such acts just empowers them to do worse.Then why do we still have wars?
In war weapons get used and destroyed, that creates more demand than simply arming for war can.