Well, a true war story might not be believed because people have certain preconceived notions or expectations about war that don't match the actual experiences. Also, the details might be so intense and brutal that the mind has a hard time accepting them as true.
It's often because the reality of war is so extreme and hard to fathom that it seems almost unbelievable.
The main reason could be that a true war story can challenge our common understanding and beliefs about war. Sometimes, the truth is so complex and overwhelming that it's easier to think it's not real. Additionally, the way the story is told or the person telling it can also influence whether it's believed or not.