Why, well mainly because such software are quite useless in the face of reality. Even industry experts have said that the way things work is that a virus is detected, dissected, a cure is developed, tested(hopefully), and then implemented, all the while the virus has spread to millions of computers all overt the internet. There is no such thing as an anti-virus anymore, just intrusion prevention systems that combines all the stuff you need to protect against intrusion or hacker attacks. Anti-viruses are so things of the past and the name serves no purpose if the software makers themselves say, there is no one perfect protective suite?!?
Anti-viruses worked better when files(and the viruses they surely contained) were transferred from computer to computer through physical media, allowing the software to check the files before they were read and used on another computer. Gone were the days of single diskette software and in with space guzzling ones that seems to always paralyze your laptop/desktop, eating up not only space but precious memory other applications so require. Take a look at current virus lists and you’ll get dizzy just counting them(millions). You pay at least a few dollars for the most basic versions and almost a hundred for some top ones but still they say no one cure works?!!