I can play physical copies of music and movies wherever I happen to bring them. Why can't I do it with the digital variant?
Largely I feel like the response to this is a rephrasing of 'because no one will be able to monetize the creation of entertainment'. But that's not a moral reasoning, that's a choice of how to foster a market. Which undermines the explanation of this being about piracy. We can try other ways of growing a market that doesn't inhibit an intuitive natural urge to share.