These models aren't doing anything different than what humans do as we grow and learn over time. Everything we think, say, or write is constructed in the same way. All of the content we've encountered through our lives create concepts that we use to derive our own thoughts and shape our thinking. The reason we don't up quoting lines or repeating the same phrases is due to the amount of material in our training corpus, which is why these models are making such a splash right now. It's the first time we've seen a large enough training set that the inferred output isn't just regurgitated lines and phrases, it's genuinely new content based on everything it was taught. That's not plagiarism.