This may not make any sense to others, but I think that a lot of the reason these guys looked so good was because of the black and white photography. I know it doesn't change their proportions or body shape, but the lack of color gives their physique a certain dimension and aesthetic that color photography loses. Sometimes color photos are a little too "real" or "perfect". Plus there is just a slight graininess to those old photos and video that helps to obscure any slight flaws and unevenness, and lets your mind fill in some of the details.
Not that those guys didn't look phenomenal. They had physiques that have stood the test of time.