It was last year that two books by commercial houses ended up with the same piece of stock art as their central image (a woman in shadow with a high ponytail, IIRC). It was such an unusual occurrence and such a big deal to the publishers/writers that it made the news.
It's so common with vanity/self pubs that it's no big deal there.
A commercial publisher wants a book cover that will stand out and get noticed. They don't want someone to go to a bookstore, or on line, knowing only that the book they're after has a certain look and have any chance of finding the wrong one.
If someone went to find that book "Mental Ward", but couldn't remember the title, they could instead ask someone about "That book with the hands like they're on the other side of a window trying to break out..." - all of those books fit the bill.
Compare that to something like, say "Wings" -- "That book with the two flower petals that look like wings."
The image in the 2nd is iconic, and specific. No other publisher would copy that image because it's too distinct.