Well, the obvious answer is that, like most WWII veterans, most of them have probably passed on.
But to learn specifics about this film, the great show it was based on as well as the people who created it, RUN to your bookstore or Amazon.com and order "The Songwriter Goes to War," by Alan Anderson. Anderson was the stage manager for the stage version of "This is the Army," and he also appears in the film. The forward was written by Irving Berlin's daughter. I just got my copy today and its wonderful.
Thank you for mentioning The Songwriter Goes To War! I just finished reading it. I've recommended it to my parents for their book club.
I enjoyed the entire book, but I'm really particularly taken by one bit of info -- "the Buick." And just think -- Berlin schlepped the Buick everywhere he went. Otherwise, he couldn't have written any songs while they were on the road.