There are two reasons a virus scanner is not sufficient protection against a hack.
Criminal hackers test their hack against commercial virus scanners to make sure their hack is not detected.
Most virus scanners use pattern matching to detect known viruses, not new ones. Hackers develop new ones daily.
People who install networks and build websites do their very best to make them secure. However, a criminal hacker has only to find one weakness to attack your business.
A Pen (Penetration) Test is your best protection. Using different techniques, I can locate vulnerabilities in three areas: web applications, networks and your workforce.