Well, why shouldn't they have to pay artists a license to use their work, especially in ways that could drastically affect the market?
There is a thing called copyright, and the exception to that rule is called fair use.
Artwork is copyrighted by default and, under the law, to use someone else's copyrighted works requires a license (that is usually bought). Whether AI training counts as fair use is the question and ultimately that is the point that will need to be proved/justified.
So again, what makes AI "fair use" and why shouldn't companies have to pay a license for their use of copyrighted artwork?