Why do studios use Roman numerals in the copyright notice in the end credits?

Readability and Internationalization

Movies are mostly produced on reels of film (few are digital). Physical film can degrade over time making it difficult to read numeric values. Some numbers will look exactly the same, for example the number 6 will become 5 when the film becomes old. Where as, roman numerals are easier to read when the film degrades. A lot of television shows are also shot with film.

The other problem is languages. Arabic numbers are hard to distinguish in fine print. Two and three look very similar. So the film industry started using roman numerals to ensure the copyright date was always readable.