Why are viruses not classified into one of the three domains?

Domains of life:

1. All the organisms are classified into three domains Archaea, Prokarya, and Eukarya.
2. Viruses do not contain any life form.
3. They are acellular and only contain a capsid made up of protein and a genetic material either RNA or DNA.
4. They require the machinery of the host cell to reproduce.
5. They do not possess any similarity with the other cells. They do not have any organelles and fail to perform metabolic activities.
6. Therefore, viruses are not placed in any domains.