An interesting read...
35mm film, the kind of film used for most old movies, can easily be considered around 20 megapixels or greater in resolution. The lesser used but absolutely enormous 65-70mm film has, as you’d guess, roughly double the potential resolution of 35mm film and could be converted into a 30-40 megapixel image. Coincidentally Ben-Hur, the movie you referenced, was shot on 65mm film.
Given that high quality 35mm film shot with quality gear can yield 20 megapixels or more of resolution when scanned with high-end equipment it becomes readily apparent how it’s very easy for movie studios to go back and, assuming they’ve preserved their original negatives properly, completely remaster a film to look absolutely amazing compared to what they released on VHS in the 1980s and DVD in the 1990s.