I think the point is that the cover is never guaranteed to accurately represent the book.
Quality of cover =/= quality of book
Yeah, the point of a book cover is to sell the book…
The point of a book cover is to cover the book.
No. Some are richly designed to showcase the book contents and others are not. That’s the entire point! It’s not the books with fancy covers that are always the best. You could find a plain cover copy of The Hobbit in your local library next to another copy that is oversized with a gold-embossed cover and an amazing painting showing the party of 14 plus a Wizard huddled on a mountaintop against the storm…
…and they’re still the same book.
My copy of The Hobbit is really weird it’s just leather and says The Hobbit in gold inset writing.
Absolutely nothing on the back, or even a barcode.
Really old books tend not to have covered designs that seems to be a relatively modern phenomenon.
It wasn’t when the idiom was coined. Have you seen hard-bound books from the 19th century in libraries?