Plagiarism plots do exist, it's just that (maybe this is a consequence of only reading quick transmigration) basically all of it is plagiarising stuff from another world, so it never has any consequences for the Author. It's still stealing someone's hard work, sure, but it won't hurt them at all, so usually it isn't as portrayed in this book. Although, as previously pointed out, basically all the books SL travels to are bad, so it's unsurprising she's like that.