Why is it so hard to treat or cure viral infections?

Because of their high rate of mutation. They also can be hard to track because they infuse their DNA into your body cell DNA and can lay dormant and undetected for years.

Technically viruses are not living as they do not contain any of the membrane bound organelles (mitochondrion) as well as ribosomes, which are present in cellular life forms; viruses contain nucleic acid (either DNA or RNA). All antibiotics invariably act by interfering with the prokaryotic 70S ribosomes during protein synthesis; but antibiotic has no effect on the eukayotic 80S ribosomes. So during viral infection, when person’s general immunity is low they are prone to infection by prokaryotic organisms (eg Staphylococcus aureus); a course of treatment with antibiotics kills the infecting bacterium, unless it happens to be resistant to that antibiotics – eg Methicillin-resistant Staphylococcus aureus, (MRSA) a common hospital bug.

It's speed of rotation.