The security is defense in depth. There are now:
- Background checks cross-references with photo ID before flying (e.g. the do not fly list)
- Enhanced security screening (there’s YouTube footage of a totally incompetent pat down of one of the 9-11 hijackers)
- Locked and strengthened cockpit doors
- A chance of an Air Marshal on a flight
- The knowledge that passengers will fight back
- Strict no-fly zones around sensitive areas like DC