Why are viruses not included in the three domain system?

Viruses are not included in the three-domain system because they do not exhibit the fundamental characteristics of living organisms. They lack cellular structure, do not possess ribosomal RNA (rRNA), and cannot carry out metabolic processes independently. Viruses require a host cell to replicate and reproduce, which places them outside the scope of the three-domain system that classifies living organisms based on cellular and molecular features.