Why don’t we just label it “want of moral fibre” like the hardcore Brits did to victims in WWI. PTSD is a much better descriptor given that a lot of the suffering according to more recent research comes from the breach of the social contract between suffering vets, who had horrible things happen in combat and the military and civilian societies around them.
Whether called PTSD, Gulf War Syndrome, shell shock (WW II), soldier's heart (WW I), neurasthenia (19th and early 20th century), and DaCosta syndrome (US Civil War) all are likely (in my opinion) due to magnesium deficiency and persistent injury to the central nervous system, from concussion, neurotoxicity, or pathogens.
The traditional military attitude toward PTSD and its kin reflects the grim realities of combat. Since most men function well enough to fight, there is no sound or fair reason on which to excuse from combat those who have the shakes without a clear and verifiable physical wound, injury, or other impediment.
Doctors do little better when they are unable to diagnose and explain an illness. Whether PTSD or Chronic Fatigue Syndrome, in clinical practice, the risk of malingering tends to make doctors reluctant to make a diagnosis for which they have no clear objective test.
Once upon a time, multiple sclerosis was in that category. Many doctors regarded its early stages as malingering. Only after years of research was the disease understood well enough to be capable of diagnosis. Its cause remains unknown, although one or more pathogens seem certain to be involved.
In application, the social contract is a tricky thing. The interests of the healthy who work, sacrifice, and suffer cannot be disregarded. After all, even if PTSD and similar ailments are real, they are hard to distinguish from well-informed malingering. This creates a risk of inviting and facilitating fraud for the sake of disability benefits. In the end, medicine is never just about the patient.