Because the moment life first appears, it multiplies and fills all easily available niches, and any new life is outcompeted by the more evolved pre-existing organisms.
We don't know where exactly viruses came from, but one of the leading theories is actually that they are the descendants of bacteria which outcompeted other bacteria. When a parasitic bacterium sheds more and more of its own metabolic capabilities, because it's more efficient to steal those abilities from its host cell, it evolves in a way that looks more and more like a virus.
"More evolved" or "more advanced" in this case doesn't mean "bigger" or "more complex". You can say viruses are equally evolved as bacteria, meaning they've spent equally long being optimized for their ecological niche.