Because it will be titled call of duty and make way more money than an obscure war title.
I think the only way to justify the title is if the movie starts in modern times with a 17 year old kid playing late night COD with his friend on a basement couch. He's so into it, he comments "I wish I could fight a real war!"
Then boom. News reports come in. There's been an attack on the western seaboard. The president (not Obama, but clearly a stand-in for him) announces San Francisco has been obliterated, and solemnly requests Congress declare war on China and Russia. The draft is reinstated, and off to war our vidya enthusiast goes.
Only it's an awful war. Waves and waves of people dying. The movie is just endless suffering. The kid's friends die gruesome unheroic deaths. His whole platoon is destroyed, and he's left fleeing for his life before we hear a snap in the bushes. He turns around, only to see an enemy fighter pointing a pistol in his face, and crack!
The gun goes off, and the kid wakes up. He fell asleep playing video games and woke up to his friend loudly hitting his knee on the coffee table.
Stretching, he rubs his eyes and asks "where are you going?" His friend replies, "the bathroom. Duty Calls."
