We don't. Nobody got prosecuted for downloading movies. Because that isn't illegal.
Whats illegal is distributing copies to other people.
Its OK though. Its a common misconception that internet piracy is illegal.
Only the distribution part has ever been successfully prosecuted.
> They didn’t do that. They used stolen copies instead.
Using stolen copies isn't illegal. It is completely legal to use other people's work without their permission. Distributing copies is the only part that is illegal.
> if we just throw copyright law completely out of the window at the same time.
Nothing is being throw out the window. The facts that I have described have always been true.