Because they absolutely require the resources of a host cell to reproduce and normally are harmful to the host.
Viruses are obligate intracellular parasites meaning strict parasite Why ??
The answer is very simple they consist DNA or RNA never both with a protein coat. They totally lack the metabolic machinery so for their replication they need metabolic machinery which is provided by host. They take over the charge of the cell and order the cell to make their progeny.
Hope the answer to the question.