Fundamentally, the problem with plagiarism isn't about whether someone else will ever make money off of my creative works. That's the main purpose of copyright, but plagiarism isn't just a problem of copyright.
I've written something that other people have liked so much that they've copied it without telling me. In and of itself, that doesn't upset me - it's flattering to have created something of enough value that anyone would want to copy it. It also doesn't upset me per se if they don't credit me for the work I did - if they want to note "Anonymous," for example. The problem comes when someone else tries to pass off my work as their own - when there is no note at all, and the implication is that they did the work themselves.
So it isn't credit that I want. It's miscredit that I don't want. And the best way to avoid miscredit is to insist on credit. That's what attribution is really about. It's a subtle distinction, but I suspect it's a significant one for a lot of people who create things, at least down here in the trenches of creative works that don't make a profit and/or aren't ever intended to make a profit. Once money enters into things, we go back to the conventional copyright and trademark laws.