To make it short, theft. Even now, on a quick look, of the 16 fictions visible on the recent series and trending lists, 8 use someone else's artwork without permission. That's half of them. Saying "I took the picture from google images" in the description doesn't somehow make it okay. It's a copyright infringement, and unfair and damaging to the artist. I've seen some authors here make big bucks each month through patreon, using gorgeous but stolen artwork to promote their stories. Covers do sell books, as everyone well knows, but even a child should also see why it's wrong to make bank on another person's work.
Strangely enough, the people running this website have decided not to care or do anything about it. Even though the website owner can be held legally responsible for the content hosted on his/her site. Besides the original artists and the young users who aren't aware of what they're doing, this is pretty bad also for those who work hard to make their covers and post legitimate works among the heaps of frauds. Amateur covers can never rival professionals in terms of visibility and popularity, even though the professionals who made those covers never allowed their imagery to be used.