And bacteria self-replicate, whereas a virus needs to infect a cell and be reproduced by that cell. Some antiviral mechanisms attack the reproduction proteins that the human cells use, which the virus cannot do without. And the human cells don't have reproductive pressure to replicate viruses, quite the contrary.

2) To further illustrate, some viruses have been nearly eliminated with a single vaccine. Polio didn't manage to adapt before going almost extinct. And a good reason why is:

3) Viruses can only evolve inside contaminated hosts. If you find a cure that stops quickly the virus from multiplying and contaminating, you are also curtailing its ability to adapt. A contaminated host is a giant casino machine, allowing the virus to mutate until it hits a new evolutionary step. A strong enough vaccine or treatment is like throwing out the virus before it has time to play much.

But so what? Anti-pathogen drugs are useful in the period during which resistance hasn't become universal, and if and when it comes a problem, we'll have other drugs.

Besides: sometimes you get lucky and the virus goes extinct before it can develop resistance (e.g. smallpox)

In fact they're so absurdly specific that while you could bathe in a solution of them and not get sick, they also frequently fail to infect slightly different members of the same species, which is why ultimately they never become antibiotic alternatives: having the right one on hand ranges from difficult to impossible.