Again, there are firewalls and there is virus software. That's one of the aspects of a good security system. But that isn't the whole of the problem. The overall point is the issue of passwords themselves. It's typical of anti-virus software and firewalls that you're always one attack behind.
Intrusion detection is the same.
They're important technologies and important parts of the security chain. But passwords are going to be vulnerable, even just sniffing over the network, which is where you want to get into more sophisticated authentication technologies like digital certificates and hardware tokens. It's the same way you can never build the uncrackable safe in the physical world. There's always a way. It's a matter of how valuable is the thing you're protecting.