If it wasn't, then buying a single copy would legally allow you to sell an infinite amount of copies.
Because that's what actually owning software means.
This. Is. A. FUCKING. Lie.
If I buy and own a physical copy of a book. I am not then entitled to set up a printing press and sell my own word-for-word copies of said book. Same applies to ownership of digital goods.
If I go to GOG and buy a DRM free game (aka I would then own that copy) I still wouldn't have the legal right to distribute copies to other people.
That's because copyright laws exist, and they apply to both situations I described. Shame on you for mindlessly repeating industry lies.