Why Are Viruses So Dumb?

In November of this year (ed: 2002), SARS was first diagnosed in Guangdong province of southeast China, then quickly spread to Hong Kong and, by February of 2003, had spread worldwide. By early July 2003, the World Health Organization declared the SARS outbreak contained, primarily due to public health measures. The entire outbreak caused over 8,000 cases with about 10% mortality.

Viruses have probably existed since life first evolved. Viruses are submicroscopic, smaller than even bacteria and other single-celled flora and fauna. Scientists have found virions (independent virus bodies not inside a living cell) in almost all ecologies and environments; they estimate there are millions of virii that we have yet to encounter, study, and catalog.

Viruses reproduce by invading a live cell of their target species and then forcing it to generate many copies of the virus. When the cell ruptures, the copies spread to other cells. Eventually, the host either dies or, in the case of animals, develops some antibody response to eliminate the invading virus. In either case, the virus eventually dies in that host.

My question is this: why haven't viruses evolved so that they co-exist with their hosts instead of killing them or forcing them to eliminate the virus? There are some beneficial virii*, but this seems like a side-effect of the natural infection cycle; it is only coincidental that some of these are helpful to humans. Why haven't viruses that help their hosts become more prevalent? Why haven't harmful virii died out?