Verified answer

Because viruses mutate very easily and often.

Once they have designed an antiviral, the virus has mutated (changed) and the drug is not effective.

Because there aren't very many antivirals, and none of them work real well.

Bacteria are incredibly complex. Less complex then us, but way more complex then the computer you are using. They are also very different from us, and there are things almost all bacteria have in common. Come up with a drug that sabotages one of these details and it will work great against many bacteria. Penicillin sabotages peptidoglycan, which almost all bacteria use (and our own bodies don't).

Viruses are very simple. There is nothing that ALL viruses have in common. Most of the machinery that builds new viruses is our own cell's machinery. Fewer potential targets for drugs. When you do come up with an antiviral drug, it usually works on only one kind of virus.

Nowadays, if drug companies devote many millions of dollars to researching one virus, they can come up with a marginally effective antiviral against that virus.

Doctors don t prescribe them because viruses use a host cell to grow and multiply. They use the host s metabolic machinery to produce viral enzymes and components. So it becomes extremely difficult for an antiviral to target only the virus without damaging the host cell. Therefore, they are too toxic for clinical use.