It is no more "theft" than you are.
I'll never get over how many people I watch online complain about how they'll never use AI because "it's theft", and then post photoshops they made with pictures they don't own, when that's not what the AI is doing.
I'll never get over how many artists I've seen complain about how AI is theft, and then paint something with "inspiration images" sitting in front of them while they paint, with their painting effectively being a blended composite of their inspiration images - when that's not what the AI is doing.
I'll never get over how many writers I've seen write the exact same derivative stuff that they also read, down to the same phrasings at times, just packaged in a new plot with new characters, and yeah, same story.
Even a person who isn't *directly* copying things that they're literally looking at is still the sum of their experiences. And it's rather hard to say that the breadth of human experience is broader than an LLM (whose "breadth of experiences" is "the whole world's outputs"), outside of the things that relate directly to having a body in a way which a blind / deaf / quadraplegic / whatnot person wouldn't grasp well.
And on that latter note, most people underestimate how well e.g. the congenitally blind actually grasp colours and the like. They're far better at reasoning about colours - similar to the sighted - than they are at knowing what colours things are. One study I read for example asked about polar bear fur. A good fraction of the congenitally blind subjects answered that they didn't know what colour it was. When asked to guess, about half of them answered that it was white so that it could blend into the snow, while the other half wrongly guessed black, but did so on the assumption that they'd want to soak up light to help stay warm. And actually in reality, both are true - to an outside observer, the exterior scattering of visible light without pigment makes them look white, but it's also designed to trap non-visible light up against black skin to absorb it for warmth. A sighted person, just seeing "white fur" and not knowing the latter property, might not have thought to even consider that.
To a LLM, our bodily experiences are akin to a blind person asked about colour: only knowing for sure things that they've learned about them directly, but still quite adept at reasoning about them.