Because copyright is not the same as stealing a physical item, so if you want to discuss the morality of it, then length does play a role, as it should since all works should enter public domain at some point, therefore ALL discussions about copyright should address the ever-extending length.
Also, discussing the morality of copyright infringement is also important, as copyright infringement does not rob the owner of the item itself. It only duplicates it.
Because a copyrighted work is not actual property, even if they call it "intellectual property" today, but that's mostly a misnomer. A copyrighted work was supposed to return to the public domain and benefit the whole society, not just the creator, by using it and improving it.
Also, discussing the morality of copyright infringement is also important, as copyright infringement does not rob the owner of the item itself. It only duplicates it.