Airport security pre-9/11 was pretty horrendous. Hijackings were surprisingly common compared to today, but the difference was that it was usually for a ransom and/or demanding that the plane go somewhere else. This led to a general policy of just going along with hijackers and letting the authorities deal with it later. The most famous example of this kind of scenario is D. B. Cooper.

An important thing to remember about Aviation in general is that every lesson is written in blood. This leads me to PSA 1771. In 1987 a disgruntled former airline employee got on plane, shot his boss and the flight crew, then intentionally crashed the plane killing everyone else. This incident was entirely caused by the poor security practices at the time. The perpetrator's employee ID hadn't been taken back when he was fired and because he had those credentials he was able to get on the plane without being searched.