The theory probably revolves around there not being a transfer of the file as much as a copy paste of it.

Physical media can be copied but you need blank physical media to copy it to, in a purely digital eco system both the buyer and seller end up being able to have copies of the sold product, which effectively treats the seller as if they are a vendor of the product being sold.

Basically in a world built on copyright law, being able to buy and sell digital media the same as physical media looks a lot like someone scalping the original product to cut into the maker's bottom line. Megacorps eating it is pretty dope but it significantly diminishes an individual developer's ability to profit off their own work as well, especially when software development already encourages so much copy pasting to make software that should be working into software that is working.