Presumably because it can't act? There are two relevant paradigms to consider:
- Diamonds move through the earth (since the earth itself is not static), grow, and split. This means they move and reproduce. Are they alive?
- Prions are proteins that reproduce directly, by bumping into other proteins and forming them into prions. Are they alive?
The general run of thought that I'm familiar with is that bacteria are unambiguously alive, viruses are ambiguously alive, and diamonds are unambiguously not alive. Prions would go somewhere on the scale between viruses and diamonds. I would personally rate the monster below prions in terms of "being alive"; it doesn't surprise me to see it called "pseudo-living".
Anyway,the question of "where is the line when life begins" is subjective and scientifically boring. It's only interesting philosophically to people who don't understand the concept of spectrum/gamut/continuum. The interesting question is where specimens fall along the spectrum of completely inert to complex life.
Viruses are mechanically simple enough that the conceptual toolkit of chemistry is enough to understand them. But the conceptual toolkit of biology, while not necessary, can still be fruitfully applied to them. (Biological understanding is also necessary for explaining why viruses might do what they do.)
There is no "not alive" category; that is the long-discredited philosophy of vitalism. It's purely a question of how you choose to do your analysis.
But I'll leave you with a selection from the wikipedia article "Virus":
Opinions differ on whether viruses are a form of life, or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes, evolve by natural selection, and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism, and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell – although bacterial species such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection.
- Diamonds move through the earth (since the earth itself is not static), grow, and split. This means they move and reproduce. Are they alive?
- Prions are proteins that reproduce directly, by bumping into other proteins and forming them into prions. Are they alive?
The general run of thought that I'm familiar with is that bacteria are unambiguously alive, viruses are ambiguously alive, and diamonds are unambiguously not alive. Prions would go somewhere on the scale between viruses and diamonds. I would personally rate the monster below prions in terms of "being alive"; it doesn't surprise me to see it called "pseudo-living".