The cousin of that argument is you can't copyright a media change. Otherwise I'd own the copyright on a ripped CD/DVD. Or I'd own the copyright on anything I have ever photocopied. Or for that matter, a printing press owner would own the copyright of anything he ever printed, which is certainly not the case.

(edited to add, another good example, is if media changes could be copyrighted, every academic paper with a quotation I've ever written would mean I now own the primary quoted source, which creates a weird chicken and egg for plagiarism where I don't need to cite myself as the source of my own words, so I need not cite anything I quote because I'd own it as part of the act of quoting it... so you can not have plagiarism if you can copyright media changes)

First, you are ignoring that derivative works have copyright from both the original author and the person making the creative alterations.

Second, plagiarism is an ethical standard that exists independent of law. It does not need copyright to exist. If you didn't write it, you quote it. Even if the original writer assigned copyright to you and you alone.

For the record: copyright in the US is largely about creativity. Other jurisdictions can and do vary (England & Wales have a "sweat of the brow" test).