You're saying that a copy of a digital thing is the same as the "only" of a physical thing. But that's not true. You can't sell grain twice, but you can sell a movie many times (especially when you account for format changes, remasterings, platform locks, licensing for special usecases like remixing, broadcasts, etc).

You'd have to steal the author's ownership of the intellectual property in order for the comparison to be valid, just as you stole ownership of his crop.

Separately, there is a reason why theft and copyright infringement are two distinct concepts in law.