If you saw a sign like this one in a store window, and you hoped to get a job in the store, but the sign did not include the archaic "inquire within" phrase, would you do anything other than inquire within?
If you saw a book cover with one person's name on it, but the text did not include "by," "By:" or "Written by," wouldn't you assume that the name is the name of the author?
A phrase like "Written by Stevie Jones" may be forgivable on a report written about Abraham Lincoln by a child in third grade, but DOES NOT belong on a book. It instantly brands the book as amateurish, and the book may be dismissed even before the cover is flipped open.
Publishers should know better. Authors should know better. Any cover designer who goes along with an author's request to include the childish text -- or puts it there without being asked -- has no business designing book covers.