> is similar to the motivation for using existing words

I don't think it's like that. If we take music, for example, the existing word would be a note or a scale or a musical instrument or a style, but a melody would be an existing sentence. As for sampling, there is creative usage of samples, like Prodigy for example where it is difficult to even recognize the source.

Also today there is some leeway in copyright enforcement. For example, I often see non-commercial amateur covers of commercial songs and the videos don't get taken down.

I put it to you that same difference. These matters of degree are what copyright lawyers haggle over. It implies to me that the whole edifice is forced into being, for its desirable (?) effects, and has no concrete foundation. Nothing pure and elegant and necessary about copyright.

Well, you asked why, anyway, and there's why: it's a natural thing to do.