Where do you get the idea that there's no economic opportunity cost without physical items? The whole idea of opportunity cost is intangible -- it's the differe

Where do you get the idea that there's no economic opportunity cost without physical items? The whole idea of opportunity cost is intangible -- it's the difference between what you got by doing what you did versus what you could have gotten by doing something else. Distributing a book may be free, but writing a book requires maybe years of effort. Why are you giving that author a free pass for not spending that time building wells and schools in the South Sudan?