Why some computer viruses refuse to die

One past incident I had: I was working as intern in a tech support company.

We had there some 1.44mb discs with the game "Lotus III", so whenever we were bored we would play a little.

One day I arrived at work... And they were scrambling all over the place using ancient antivirus from same era as Lotus III... Found out we played that game on tons of random computers and it was infected with a DOS vírus that could actually copy itself to USB drives too... Whooops.

We caused outbreak of what was then an already obsolete virus. (Most of the machines had windows 98 in them at best... The virus worked on then surprisingly fine despite being intended for DOS, and in those machines specifically it would infect USB)

It has always seemed like an unending battle to me. The only way this ends is to make an AI antivirus that can decide whether a particular software is malicious or not.