Why are viruses unable to perform any metabolic functions independently?

Viruses only have their genetic material enclosed in the protein coat. Viruses are considered to be non-living when they are present outside their host because they can not replicate outside.

Viruses are inactive outside the host body and require host machinery to replicate which means when virus genetic material is entered in the host cell then it uses the condition of host cell to express its enzymes and protein.  

These enzymes and proteins help in the replication of viral genetic material and formation of proteins of capsid in which its DNA can fit. Viruses can not perform these functions independently because it does not have any cell organelles like nucleus where replication takes place and ribosome which helps in protein synthesis.