A premade book cover is a newer way for authors to get covers for their books faster.
Instead of hiring a cover designer and going through full design process (exchange information about the book, then get first design concepts, then provide feedback, then get the revision etc) authors just browse a collection of ready made designs that designer has made on his/her own. If the premade matches the book closely, then the author can just outright buy it or ask for a preview of it with their details (title, author name, tagline etc). The process is simple and usually done by emails and often takes about 24-48hrs to finish.
With premade covers, designers usually have many designs available. Some have hundreds, few even have thousands of designs on sale. Many premades are rejected versions of custom covers from old projects (the designs author didn’t approve for final design). It makes little sense to delete them when with some changes they can be put on sale and serve as great cover for other book.
Usually, premades are based on genre-specific cliches to let readers easily recognize what genre the book is while also trying to have some originality to it and not be too generic. Designers usually have many designs in one genre and that allows authors to pick from many designs.
Some designers actually make premade book covers full time and don’t even need to do custom covers. It is convenient for them to come up with ideas and just create freely. Then, if there is a book that matches the idea and authors wants the cover, it can be a quick and convenient way to get a book cover.
Premades are most common and were started in self-publishing to give more authors access to great design at reasonable prices.