My grandfather was US Army Infantry in France 1917-1918. He was in the trenches until the armistice. Never spoke a word about it to anyone period. Barely acknowledged that he had been in the army. Best to let those memories die and stay that way.
That's just the thing....those memories never die. Combat vets don't refuse to talk about it because they've forgotten.
I'm sure it's just less painful for them to leave it in a lock box in the back of their minds.