Strictly speaking it wouldn't exactly be stealing, but I would still consider it as about equal to it, especially with regards to economic benefits. It may not be producing exact copies (which strictly speaking isn't stealing, but is violating copyright) or actually stealing, but it's exploiting the style that most people would assume mean that that specific artist made it and thus depriving that artist from benefiting from people wanting art from that artist/in that style.

Now, I'm not conflicted about people who have made millions off their art having people make imitations or copies, those people live more than comfortably enough. But in your example there are still other human artists benefiting, which is not the case for computationally generated works. It's great for me to be able to have computers create art for a DnD campaign or something, but I still recognize that it's making it harder for artists to earn a living from their skills. And to a certain degree it makes it so people who never would have had any such art now can. It's in many ways like piracy with the same ethical framing. And as with piracy it may be that people that use AI to make them art become greater "consumers" of art made by humans as well, paying it forward. But it may also not work exactly that way.