You just aren't browsing to the right (or in this case, wrong) kinds of sites. The Internet has a vast, scummy underbelly where these things go on. I never go there, and you obviously don't either, but that doesn't make it less of a threat.
I don't know what I'm arguing about, really. Of course some people consider viruses a threat and it's good that AV program exists. I just haven't ever seen the need for it personallty. I do an occasional online scan once a year or so and nothing ever shows up.