It's theft of labor without any compensation, aimed at cheapening the very value of that labor.

A human artist can, and often does, train simply by looking at the real world. The art they then produce is a result of that knowledge being interpreted and stylized by their own brain and perception. The decision making on how to represent a given subject, what details to add and leave out to achieve an effect, is done by the artist themselves. It's a product of their internal mental laboring.

By contrast, if you trained an AI on photos alone it would never, ever produce anything that looks like a drawing or a piece of art, it would never create a stylized piece of art or make a creative decision of its own.

In order to produce art the AI must be fueled with human created art, that humans labored to produce. The human artists are not being compensated for the use of that labor, and even worse the AI is leveraging that to make the human labor worth less. And what's more, that AI's ability will stagnate without further theft of newer, more novel art and concepts.

Without that keystone of human labor the AI simply can't function.

Ripping off so many people at once and so chaotically that you can't distinguish exactly how any given individual is being exploited doesn't mean those people aren't still being ripped off. The machine that the tech bros created could not exist without the stolen labor of the artists.