Always remember that the Internet was designed to freely exchange information. All of the underlying structure was built with the goal of making it easy to find and retrieve data. Now we come along and are trying to add security to it. It was not designed with security in mind, so all that security is pasted on top, so to speak. The more complex a system becomes the more unmanageable it becomes.
Modern networks are very complex systems.
The software and hardware is always changing, and every patch that fixes one bug, can cause another one to appear. The more complex the system, the more unintended consequences you will have. Most large high tech companies have hackers working there, they just don't know it. Either a hacker is using them as a shell, or someone inside is hacking me. It is much, much easier to hack a network from inside than outside.