Astronauts are not paying customers, but part of a crew so your point is unclear. “Yet they allow anyone on board”…well, how would you go about vetting every paying customer? We already complain about the physical security measures but now you would like to introduce…mental security?
Phycological interviews? Questionnaires? or the good ol’ “She dun look funny” approach that would be oh so accurate (/s).
To be blunt, you cannot screen for potential, Minority Report is just SciFi so instead, you go with the idea that most people are not crazy, that crews are trained to deal with the few, passengers are more likely to get involved, and life goes on.
Planes flew a few days after 9/11 and they quickly filled. I flew three weeks after and knew that if any crazy person stood and threatened the flight, 80+ people would do what ever to stop it because we now knew, negotiation was dead…that was/is better then any TSA screening.
If you still worry, fly private or drive, but keep in mind that the driver next to you may go bonkers, but their won’t be anyone stopping them slamming into your car.