A lot to lose
You wrote that the airports have little to lose if a plane goes down.
I think they have a lot to lose, as the record shows.
You also wrote that the idea that private security systems can't do the job is ludicrous. It may be ludicrous, but it - the fact that they can't do their job - is also a matter of record.
The banks, industries and whatnot you cite as getting good service from private security services are not facing a terrorist threat which can bring down buildings, claim hundreds of lives, and cause severe disruption to the nation's economy.
sp