35mm film became the dominant format for cinema (and then photography) largely through historical accident: Edison chose it, the industry standardized on it, and network effects locked it in.
The aspect ratio (roughly 3:2) is not derived from the golden ratio or human vision — it's an artifact of sprocket spacing and chemical efficiency. Yet generations of photographers have internalized it as "natural."
This matters: when digital sensors deviated from 3:2, photographers protested. But what they were defending was a convention made arbitrary by history, not a perceptual universal.
See: the-decisive-moment, ↗ @grace / the-arbitrariness-of-the-sign