(BBC TV credits use Roman numerals as well. Apparently they switched from Arabic to Roman as late as 1976: https://www.tvforum.co.uk/tvhome/bbc-colour-end-credits-chan... )
Wikipedia suggests opening credits were always incomplete credit-wise - "Films generally had opening credits only, which consisted of just major cast and crew, although sometimes the names of the cast and the characters they played would be shown at the end. " - but doesn't talk about copyright there. https://en.wikipedia.org/wiki/Closing_credits
It's a group of people that you pay as a Director and that sets the guidelines how work should be done.
https://starwars.fandom.com/wiki/George_Lucas#:~:text=The%20....
I don't have a specific recollection of if they included all the "legalese" sort of non-credits like these copyright notices.
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.
It did used to be more common, especially for things like monuments. But it's become less common over time. I was traveling with a friend of mine who went to good suburban public schools outside of a major city and then an elite technical school. I was somewhat shocked to learn she had never been taught (or bothered to learn) Roman numerals. Even if it's sort of a trivia thing at this point, I just assumed it was something that a reasonably well-educated person would just know.
Edit: There was also a phase of hybrid Roman/Arabic hyphenated page numbering in there somewhere, if memory serves most of the books I have seen with that were from the interwar period, chapter/section would be Roman and page in Arabic.
Hello, fellow programmer. A fine day for zero-based indexing, isn't it?