Because by themselves, they don't have most of the properties of living things. They have no metabolism, and they do not reproduce, for example. They're kind

Because by themselves, they don't have most of the properties of living things. They have no metabolism, and they do not reproduce, for example. They're kind of like (physical) books. They can affect living things (people), and some books cause living things to reproduce the books, sometimes in large numbers. But few people would regard books as alive in the way that a dog or tree is.