So many problems with your reasoning.
"Nothing was stolen from the artists but instead used without their permission"
Yes and no. Sure, the artist didn't loose anything physical, but neither did music or movie producers when people downloaded and shared MP3s and videos. They still won in court based on the profits they determined the "theft" cost them, and the settlements were absurdly high. How is this different? An artist's work is essentially their resume. AI companies use their work without permission to create programs specifically intended to generate similar work in seconds, this substantially impacts an artist's ability to profit from their work. You seem to be suggesting that artists have no right to control the profits their work can generate - an argument I can't imagine you would extend to corporations.
"The thing being used is an idea"
This is profoundly absurd. AI companies aren't taking ideas directly from artist's heads... yet. They're not training their models on ideas. They're training them on the actual images artists create with skills honed over decades of work.
"not anything the artist loses access to when someone else has it"
Again, see point #1. The courts have long established that what's lost in IP theft is the potential for future profits, not something directly physical. By your reasoning here, there should be no such things as patents. I should be able to take anyone or any corporation's "ideas" and use them to produce my own products to sell. And this is a perfect analogy - why would any corporation invest millions or billions of dollars developing a product if anyone could just take the "ideas" they came up with and immediately undercut the corporation with clones or variants of their products? Exactly similar, why would an artist invest years or decades of time honing the skills needed to create imagery if massive corporations can just take that work, feed it into their programs and generate similar work in seconds for pennies?
"What is there to complain about"
The loss of income potential, which is precisely what courts have agreed with when corporations are on the receiving end of IP theft.
"Why should others listen to the complaints"
Because what's happening is objectively wrong. You are exactly the kind of person the corporatocracy wants - someone who just say "Ehhh, I wasn't personally impacted, so I don't care". And not only don't you care, you actively argue in favor of the corporations. Is it any wonder society is what it is today?