I have a SharePoint 2013 Document library that when creating a new view for the option under the Style section is missing both "Boxed" style options.
Any idea why they are missing and how to get them back?
It's not missing , by default the Document Library views does not have a Boxed style, only the available styles are
Meanwhile, you can use the Document Details style in document library view that working like the boxed style in list view.
Document Details Style