Because copyright law is bunch of crude analogies hacked together that used the physical encodings of information as a proxy for a creator's financial interests in a work. It worked great in the age of print when mainly you were talking about books which were cheap to mass produce but expensive to copy.
But today, conceptualizing an author's rights to a work as a monopoly on copying leads to nonsensical results. Suppose I download a song to the same computer twice, as can easily happen. Technically because the thing I did wrong was copying, I infringed *twice*; however it hardly does twice the harm to the author's interests. On the other hand if I copy that song once but listen to it a thousand times, you could reasonably argue I'm doing more harm to the author's interest than if I downloaded it a thousand times but *never* listened to it.
It's all just a way to get content creators paid; a ridiculously complex and arcane way, but it's familiar because it's traditional. You can't expect it to make sense, especially by trying to draw subtly different analogies.