Those who write viruses do so either for peer credit, getting a job, or they spite a group of people, organization, or country. Those reasons also apply for hackers as well. They can't totally get rid of viruses for the same reason they can't get rid of crime.
Unfortunately this won't change in the near foreseeable future. For one reason, many virus developers write viruses to solely attack organizations that don't align with their idealism. e.g. Microsoft. The recent Stuxnet worm that uses a security flaw in Windows .Ink files, mostly targeted Iranian computers when it first appeared.
Virus developers also build their viruses to show off to their friends to see who can write the most damaging virus or get the most publicity. It's also true that some may even use their "virus development experience" to try and obtain a job. Although I doubt they'll publicly confess to any specifics.
Read more on the Stuxnet worm at Microsoft's TechNet website: http://blogs.technet.com/b/mmpc/archive/2010/07/16...