You need to read about why RMS wrote the GPL in the first place. Yes obviously Imaginary Property is imaginary, but we have to exist within the legal system. RMS used the copyright system as a way to legally ensure the freedoms that copyright traditionally restricts, and that is why he calls it "copyleft".

Once you've reformed or repealed copyright law in some imaginary revolutionary future, you can use whatever code or material you want in your work and publish it freely as a derivative work, but until then you have to acknowledge the existence of the legal frameworks that we live in.

If we start filling our own FLOSS-licensed codebases with code of unknown provenance we open ourselves to legal challenges, it makes the entire license unenforceable, and it destroys the share-alike ecosystem that we've built over the past 35 years.