Because:
They aren't cells, well not like animal and plant ones. They are just DNA/RNA surrounded by protein
They have no metabolism, they have no energy needs, which makes them unlike living cells
They produce no waste, no energy going in means no energy coming back out again, which is also different to living cells
They don't reproduce by mitosis/meiosis, they can only replicate inside other cells
They don't grow, they stay as they are from birth until death
They don't respond to outside stimuli they just live their happy little lives, oblivious to everything around them
