It’s a Swiss cheese model of risk management.

Alone, each layer (slice) of the security system has holes. But, when you put more layers on top (each with their own holes) some of the holes are now covered. Add more layers, and cover more holes.

In an airport, it is not just TSA but also cameras doing pattern recognition in the airport for suspicious activity, add into the mix complex national security measures and you would hope there is sufficient layers/coverage to stop most serious cases.