Why hasn't anyone written a virus that can get around anti-virus software?

I wouldn't imagine the top 5% of programmers would be working at Norton or McAfee.

Why hasn't anyone written a virus that can get around anti-virus software?adware

They do daily but AV companies patch their data base daily as well. The thing is that most virus writers are simply cutting and pasting existing code ergo the volume of worm based viruses that can easily be contained via heuristic scanning.

But every once in awhile someone gets innovative and that's the real threat as it can take days to weeks to identify and crack those ugly little buggers

Why hasn't anyone written a virus that can get around anti-virus software?antivirus scan

Viruses do get around antivirus software.

The companies who make antivirus software have special unprotected servers which deliberatley get infected to detect new variants.

That way they can find fixes and generate new virus databases, usually on a daily basis.
they have.
They do all the time, that's why anti virus software needs to be continuously updated.
Where do you think they work, the anti virus softwares block the virus after the virus is released but before the customers get infected with it, that's why updates are released, this means it needs to have a virus to find the cure, the business is who release it first.
the anti virus people make it imposible for them to get around it, they have much more money for research and development to make sure that no viruses can get through