You'd be shocked at how fad-driven the book cover industry is.
Assemble a large group of paperbacks in a given genre. If you've got enough of them, you can start sorting by trends.

All text?
Bright comicbook colors?
Painted books-specific cover?
Close up a woman's face?
Man walking away on rainy night?

At the library, look at the display of new books and count how many are similar.

"A Novel" is just one more example.

And yes, traditional publishing has a low opinion of readers and writers.