You could get into the semantics of "learning" - does JPEG encoding count as the computer "learning" how to reproduce the original image? But trying to create some metric for why LLMs "learn" and JPEG doesn't "learn" on the basis of the algorithms is a philosophical endeavor. Copyright is more about practicality - about realized externalities - than it is about philosophy. That's why selling cars and selling guns are regulated differently, despite the fact that you could reduce both to "metal mechanical machines that kill" by rhetorical argument.

Even from a strictly legal perspective, it actually is fairly clear-cut. The answer to "what if I (a human) read GPL code and then reuse the knowledge gained from it..." comes down to a few straightforward properties of the license. GPL doesn't cover "reduced to practice" as many corporate contracts do, so terms covering "the knowledge gained" are lenient. GPL covers "verbatim" copies which is what LLMs are doing, that's as clear cut as it gets. Inb4: "So what if I add a few spaces here and there?" - well, GPL also covers "a work based on"; this is where I (who am not a lawyer) can't speak confidently, but surely there are legal differences between "based on" and "reduced to practice", considering that both are very common occurrences in contracts, so there actually would be a lot of precedent.

Just learning from the GPL code to make yourself smarter is not the problem.