Final answer:
Viruses are not considered living because they cannot reproduce autonomously, lack cellular structures, and do not maintain metabolism or homeostasis.
Step-by-step explanation:
The main reason why viruses are not considered to be alive is that they cannot reproduce on their own. A virus must infect a host cell and hijack the cell's machinery to make copies of itself, a process that is fundamentally different from how all living organisms reproduce. Furthermore, viruses do not possess cellular components such as a plasma membrane, cytoplasm, and ribosomes, nor do they show signs of metabolism or maintain homeostasis, which are essential characteristics of life.