I've been flying commercially for 64 years, so I have had an opportunity to observe airport security practices for quite a while. Searching passengers and carry-ons became a thing in the 1960s when hijackers seemingly routinely forced flights to divert to places like Havana. I remember one incident where the hijacker commandeered the flight with a bottle of shaving lotion wrapped in a towel, fooling the flight crew into thinking he had a bomb. In those days, the local airport and/or the dominant airline at the local airport hired private security to man the checkpoints.

The airlines started selling non-refundable tickets in the 1980s in the aftermath of airline deregulation (which passed Congress in 1978 and resulted in the Civil Aeronautics Board going out of business; airlines prior to 1978 had to apply to the CAB for authority to fly between interstate city-pairs and had to file their tariffs with them). A black market in non-refundable tickets quickly emerged. The airlines were out to stop this, and a major (if not the major) purpose of airport security was to make sure that the person whose name was on the ticket was actually the person who was going to fly. However, anyone could enter the airsides so long as they went through the metal detectors. One manifestation of this was that Pittsburgh built a new midfield terminal with a shopping mall which encouraged local shoppers to visit, even though they had no intention to take a plane trip.

That all changed in the aftermath of 9/11/01. The airlines and airports wanted out of responsibility for airport security, and Congress, wanting to show the public that they were doing something, created the TSA. The TSA made a big deal of claiming that 100% ID checks were the panacea. This merely redirected passenger dissatisfaction with airport security away from the airlines and upon the federal government. Innovations like the PIT shopping mall were destroyed. Then you have the shoe bomber, the underwear bomber and the chemical mixing incident.

Yes, we have security theater, but I still contend that the major purpose of airport security is to prevent low-cost airline tickets from being "scalped".