I’m too young to have been around in that era, but I have noticed the same thing in retrospect, just reading about various actors. It’s funny though, some of the actors with the most impressive service records turn out to be the ones you would least expect.
For example, I would never have pegged Jimmy Stewart as a “war hero” from his films, but he was drafted, rejected, then reapplied multiple times until he made the cut (after having been a famous actor for years). Then, when the Army tried to relegate him to public relations and training duties, he kept petitioning until he was put on active combat duty in Europe, lead bomber squadrons into Germany, and earned high decorations.
Jimmy Stewart was a MAN - compare his war record to John Wayne's for example.
Like you said, Stewart actively sought combat duty and flew in some of the hairiest bombing raids of WWII - I'm almost positive he was on the schweinfurt raid. He was the first major star to put on the uniform in WWII when he enlisted and he continued to fly as a reservist long after WWII. He even flew on a B-52 mission in Vietnam. He ended his military service as a brigadier general.