But old films still featured recognizable human bodies and human faces—bodies that could theoretically be achieved by a single person without the aid of a team of personal trainers, dieticians, private chefs, and chemists.

Well, but when those bodies we not the absolute epitome of military fitness, would you believe them doing the feats that they do in the movies.
Sure it’s not 100% believable even now, but it kinda feels more “fine” if someone with stone-like protruding abs and pecs, takes a pillar to the torso and gets pushed back 50ft instead of getting squished.