Jeff is your average network administrator, responsible for Acme, Inc.'s Microsoft-based corporate network. With a rubber bladder, the gas tank would have been much less likely to puncture during a rear-end collision and cause the Pinto to burst into a fireball. Now in virtually every modern car there is a rubber bladder in the fuel tank. More or less identical situations happen all the time in computer security.
Protocols are unsecure because they lack a simple authentication or encryption method.
SNMP, for example, has earned a reputation of being an extremely unsecure protocol. If you read the history of SNMP, you will find that security was supposed to be built in. However, the vendors never quite got around to formalizing any standards regarding security. It's easy to make horrendous mistakes. Even with technology available for avoiding them, failure to use that technology can lead to disastrous results.