At Cosmos, science writer Jake Port offers reasons, including
In order to replicate, viruses must first hijack the reproductive equipment of a host cell, redirecting it to ‘photocopy’ the genetic code of the virus and seal it inside a newly formed container, known as the capsid. Without a host cell, the virus simply can’t replicate.
Viruses fail the second question for the same reason. Unlike other living organisms that can self-divide, splitting a single cell into two, viruses must ‘assemble’ themselves by taking control of the host cell, which manufactures and assembles the viral components.
Finally, a virus isn’t considered living because it doesn’t need to consume energy to survive, nor is it able to regulate its own temperature. Unlike living organisms that meet their energy needs by metabolic processes that supply energy-rich units of adenosine triphosphate (ATP), the energy currency of life, viruses can survive on nothing. In theory, a virus can drift around indefinitely until it contacts the right kind of cell for it to bind to and infect, thus creating more copies itself. More.
This argument has a long history (see below). The problem is, the fact that viruses seek to replicate themselves at all would put them on the side of life.
See also: Another stab at whether viruses are alive
Phil Sci journal: Special section on understanding viruses
Why “evolution” is changing? Consider viruses
The Scientist asks, Should giant viruses be the fourth domain of life? Eukaryotes, prokaryotes, archaea… and viruses?
Are viruses nature’s perfect machine? Or alive?