This. You get to see old cities with a lot of commemorative monuments, and they are all with years in Roman numerals (and many of them, with the whole inscription in Latin).
Academic books were written in Latin and were dated with Roman numerals because, well, they were written in Latin.
For example here's a cover of Calvin's Institutes of the Christian Religion, 4th Latin edition, printed in 1562, with the year of edition in Roman numerals:
Academic books were written in Latin and were dated with Roman numerals because, well, they were written in Latin.
For example here's a cover of Calvin's Institutes of the Christian Religion, 4th Latin edition, printed in 1562, with the year of edition in Roman numerals:
https://archive.org/details/institutiochrist1592calv/page/n3... (see page 4 of the book browser)
Films, as creative work, simply followed this tradition. That's it.