These reasons may seem true for you personally, but I disagree strongly with both as generalizations, and they certainly aren't true for me.
They're also both kinda-sorta saying the same thing, re: ownership and that being the primary sticking point, creatively speaking.
My more-than-two-cents:
I think of my mixes as my own AND as collaborations with the original composer's ideas. I don't think it has to be one or the other. I actually tend to think of ALL artistic creation as a collaboration of sorts, even the most personal works, so it all lays on a spectrum... not a Boolean. So this is a false dichotomy, to me at least.
I've heard hundreds of arrangements that brought original ideas to the table, relative to the source in question; a musical work's status as an arrangement carries SOME obligation to reuse ideas, motifs, etc., but within that framework artists can be quite creative, as we've seen over two decades, and outside of that framework, making something original, artists can still be quite derivative. So again, the premise seems weird to me, too much hinged on some notion of personal ownership being the end-all be-all & offering a stamp of great meaning & significance.
My answer is that I still do it, primarily because there are still plenty of melodies out there that I want to hear a certain way, that I can bring something to, and make something of worth that I enjoy and that others might enjoy as well.