I believe that antivirus software, as massive of a market as it is, exists purely because of major, long-running design and philosophy flaw in Windows.
Software DistributionThe approach to software distribution is to force the user to put trust in self-extracting binary packages, many of which are only available from a single source and never audited by a trusted party. This trust requirement, be it implicit or explicit, makes no discrimination; UAC will provide the same warning and require the same escalation of permissions regardless of what is installed. Occasionally, UAC will display information about the software publisher if the installer is signed, but most often this warning is ignored. This kind of process if only useful to the informed user. The average user has no mean with which to differentiate trustworthy sources from the nefarious kind.
By contrast, antivirus software only attempts to fix problems as they arise, with large and impractical vulnerability and offending software lists to work with. This not only involves a manual process more demanding and complex than the aforementioned repositories, it also requires those lists to be kept constantly up to date across millions of machines, on a urgent basis rather than on demand. Most of you Windows 10 users know that if an update process is disruptive, it becomes magnitudes more aggravating once forced upon the end user.