Because the pirate that's selling the MP3 didn't go through any of the effort to make it. They are benefiting from the work of others, without giving them any

Because the pirate that's selling the MP3 didn't go through any of the effort to make it. They are benefiting from the work of others, without giving them any credit. When the copy is free, there's a non-zero cost associated with making that copy. 20 years ago, it was the cost of a cassette tape. Even now, there is a bandwidth cost associated with giving away a copy.