Why doesn't someone create an Anti-virus?

Simply put, another virus that replicates and seeks out and destroys other viruses.

Why doesn't someone create an Anti-virus?vista

This is a good idea. But why do people not program that software until now anyway? Then there is a question and reason behind that.. Anti-virus these days are meant to protect us from virus, not attack and kill a virus. Furthermore, viruses are being created by humans ourselves, anti-virus softwares do not have the capability to foresee what virus will come out in the near future. We cannot predict what will be created and what codes in order to kill the virus because it has not been created yet. Lets put it this way, were doctors able to predict that SARS will infect humans? :) Maybe I'm wrong, but I also hope that there is an aggressive antivirus software out there that kills viruses. Ever heard of Artificial Intelligence group? I hope they could do something about this.

Why doesn't someone create an Anti-virus?antispyware

Actually, for a while, that was going on, in the rivalry between the authors of the Bagle and Netsky viruses. However, that was a limited case; most viruses are not designed to remove other viruses.
many good antivirus out now?
Antivirus is a virus which will kill the actual virus :-)
I get what you're saying. A more offensive approach than the defensive approach in todays personal security. Cool idea!