If they did, they would quickly realize that most systems are far more complex to analyze, and that there are security flaws everywhere. This is a direct result of the complexity of these systems. And everything is relevant to security analysis. This complexity isn't limited to single systems, but includes interactions between systems as well. For years we knew that Internet applications like sendmail and rlogin had to be secure, but the recent epidemic of macro viruses shows that Microsoft Word and Excel need to be secure too.
Malicious attachments can tunnel through firewalls.
Maintenance ports on routers can compromise networks, as can random modems. DSL and satellite modems can completely compromise security. So can Java or Microsoft Outlook. Or your recycling bin. The networks of the future will be necessarily more complex, and therefore less secure.