WC, when was the last time you accessed a military network? Why don't you actually try to do some reading. Heck, read up on the AFNet and its design. Also, you know no network is completely securable, right? They hack into government files the same way hackers slip in everywhere else: social engineering, exploiting firewall holes, etc etc.
The more you lock down a network, the more complexity/cost/lag is added as well. The govt has to perform cost/benefits analysis for networks too. We aren't going to throw $10k encryptors on everyone's desktop.