I might be wrong, but can a book in one country not be published by different publishers? (I'd not know how, but I have the Mars Trilogy here, via Amazon, and the first book comes from a different publisher than the 2 others)
Because then I'd guess that they just have their own inhouse artists, and do not share/sell the art between each other.
Books can be published by different publishers in different countries
and in the same country.
For example, I have three different paperback editions of
The Bloody Sun (one of the novels in Marion Zimmer Bradley's Darkover series). The earliest edition is shorter than the next edition, as Bradley revised it and added some content. Both of these were published in the U.S. by Ace. The most recent edition I have contains the same text as the second Ace edition, but it was published by DAW. All three were published in the U.S. and all three have different cover art.
I've seen different cover art again on foreign language editions of her books. In some cases I've wished the American editions could have that art.
In other cases, I have to wonder if the artist had even the vaguest idea of what the book was about. One of the worst offenders in this is whoever did the cover art for the UK editions of F.M. Busby's Hulzein novels. Rissa Kerguelen, at the time she leaves Earth, is 17 years old. Yet the cover art depicts a woman who's somewhere between 30 and 40. She isn't even 40 by the end of the series, let alone the first two books!
And there are plenty of instances where cover art is reused. The same art is used for the cover of Jo Clayton's novel
The Snares of Ibex (in the Diadem series) as in a foreign edition of one of MZB's Darkover novels.
One reason why this happens is likely as you suggest - the publisher has a stable of artists it uses.
Another reason is the same as re-issuing a book with a different title. Fool the customers into buying it, thinking it's a new one they haven't read before. This is why, if you buy your books on Amazon, you should always read the reviews. If this is indeed a re-issue of the same text but different cover art or even a different title, chances are good that someone will have already been taken in and has posted a warning to anyone else thinking of buying this book.