Because the item is a copy, and (digital) copies actually do cost nothing. That is just the physical fact, and people will always have some underlying sense of it -- as they should.
The idea that you should not be allowed to make copies, but must pay for them, is very unnatural. Humans are evolved to copy and share information -- that is the most effective behaviour. The problem is not people's inclinations, the problem is copyright/IP. You possibly can make some case that IP is a net positive system (although no-one seems to have found any evidence), but nevertheless, its essential structure/mechanism is intrinsically problematic.
The idea that you should not be allowed to make copies, but must pay for them, is very unnatural. Humans are evolved to copy and share information -- that is the most effective behaviour. The problem is not people's inclinations, the problem is copyright/IP. You possibly can make some case that IP is a net positive system (although no-one seems to have found any evidence), but nevertheless, its essential structure/mechanism is intrinsically problematic.