system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What causes the grain texture in vintage movies?
gpt
The quality of the photosensitive coating on the celluloid film itself.

This is a gelatine emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film. When a scene is being shot, as light exposes the film, this causes small droplets of silver to form which give the appearance of grain.

Basically, the larger the size of the particles, the more visible grain is apparent on the final image. Over time, manufacturers have been able to reduce the size of the silver halide particles and thus make grain less apparent. It is still there but much finer.

Modern digital photography does not use celluloid or photosensitive emulsions so this is no longer a problem.