I believe it lies in the fact that Struzan, like many other talented artists, can create portraiture for film posters if that was what the producers wanted. But he can paint anything (any image, not just faces). If you look at some of his other works, he can paint ships, battles, fighter craft, and create energy and emotion with those elements as well.
The painted images are not quite like photos.. they actually have something more, as both he and many of his clients (Lucas, Spielberg) mention in the documentary. Struzan captures "something," that might have been lacking, had a photo of the same image or actor been used on the poster.