Um...no, it can't.
I used Midjourney AI for two book covers - the books in question had sat on my hard drive for around 20 years, and I was publishing them with no budget as a "wouldn't it be nice if somebody had the chance to read this" instead of as a "tentpole" release. So, what I was able to get after a fair amount of effort was something serviceable, but also very general. So, here is AI art generated by Midjourney:
So, you get a general figure on an ice field. Happily, in this version, he has two legs (I'm not being facetious here - most of the generated art either had one leg or three). There is no real character design, and there isn't any real customization either. The image was generated and regenerated until something worked. It is good enough, but nothing more.
(And to correct the misinformation about copyright from a certain other poster, if the AI is doing a direct copy and paste from an image, then there may be a violation of copyright, if a distinct new work is not created. If it is generating an image using an algorithm drawing upon works of prior art but not copying and pasting - essentially, generating a new image in a certain style - there is no infringement. Copyright only protects specific implementations of a work, and the right for somebody else to create derivations of that work is protected in international law under Article 2(3) of the Berne Convention, which states "Translations, adaptations, arrangements of music and other alterations of a literary or artistic work shall be protected as original works without prejudice to the copyright in the original work.")
In comparison, an artist can provide a level of customization and character design that no AI could ever match. An artist can respond to feedback, and make sure that what you get is an actual representation of your work, and not just a random representation that is close enough. So, for comparison, here is the cover of the second Re:Apotheosis book:
What you are seeing are the actual characters appearing in the book overlaid over the background of a scene from The Odyssey of Daiki Yamato in which an information stream is creating a new story world. An AI would not be able to do this. An artist can.
So, there is a place for AI art (if you have no money to dedicate to a book cover and you need something good enough, Midjourney will do the trick), and I think as time goes on it will probably join the toolkit of the digital artist for things like rendering backgrounds (mountains, cityscapes, etc.). But, it cannot replace an actual artist, and if you can hire one, you should.
In fact, I would put it this way - if you have the money available, HIRE A F--KING ARTIST!