Because sometimes there's no war left to fight but the soldiers still can't return their minds to civilian life. They're disabled, unable to hold down a job reliably, unconfident in their ability to do normal tasks like go grocery shopping without having a debilitating panic attack which causes additional heart damage every time.

There's tons and tons of examples why stress after trauma is a disorder. Also a very good wikipedia article if you'd like a place to start: https://en.wikipedia.org/wiki/Post-traumatic_stress_disorder