When you enable ratings, two ratings fields (average rating and number of ratings) are added to the content types available ...

When you enable ratings, two ratings fields (average rating and number of ratings) are added to the content types available for this list. The column "Rating (0-5)" is also added to the default view. If you add new content types to this list later, and they do not already contain the ratings fields, you will need to add the ratings fields to them either manually, or by returning to this page and re-enabling ratings. If you disable ratings, the rating fields are removed from the list, but they are not removed from the content types for this list or from views that already have rating columns.