Except that antivirus software is, by definition, always playing catchup. When you hit the point that you need antivirus software, you've already lost.
Just another example of the difference between "safety" and "security". Houses in Kennilworth, IL need less security than houses in Rogers Park --- there are something like 192 police officers per Kennilworth township block --- and yet Kennilworth houses can be counted on to have state-of-the-art security, because they can, and state-of-the-art security is a selling point.
I share your take on antivirus software, but do note that preventing a user from installing something with a known virus on it does not have negative utility.
So many people these days just install anything without really checking what it is. I don't consider this anti-viral, more like a condom for the lazier users.