Why do viruses need a living cell to reproduce?

Well, look at the differences between bacrteria and viruses. Bacteria contain the genetic blueprint (DNA) and all the tools (ribosomes, proteins, etc.) they need to reproduce themselves. Viruses don't have any of that. They contain only a limited genetic blueprint and they don't have the necessary building tools. They have to invade other cells and hijack their cellular machinery to reproduce. Viruses invade by attaching to a cell and injecting their genes or by being swallowed up by the cell then the viral genes tell the bacterium to begin making new virus parts. These parts come together to make whole new viruses inside the bacterium which eventually bursts releasing the new viruses so they can continue the cycle.