Hackey, but I guess some plastic would be enough to stop metal on metal contact and prevent sparks?
Not that my Miata “temporarily” has cardboard wrapped in tape wrapped around the cold air intake pipe to prevent it from rubbing against the frame. Nope, definitely not.
Price. Once the industry retooled the production lines, CDs became dirt cheap to reproduce, and successive generations of optical media are only somewhat more expensive. Plastic shells and mechanisms cost money. CDs are probably the cheapest physical audio format ever (at least as far as production costs are concerned).