Because someone takes the work you dedicated countless hours of your lifetime to, and gains off it without any effort while you don't, no?
What is the harm to you? Not releasing it also means that you don’t profit from it.
If you’re just sitting on it and not monetizing it, and you release it and someone else generates revenue with it, you have not been harmed in any way.
humans have an inate built-in (genetic) sense of reciprocity with regard to sharing. It will decrease the quality of his life if he feels he's participating in an unfair system, even to the point of harm (stress, anxiety)