There are many who will be able to write this better than I can, but in short:

Terrorism is usually a solitary crime or at most a small group of people. In the end there are very few people who are willing to die for their cause, so simply "getting tougher" doesn't make the TSA anymore likely to find anything/anyone - there just aren't that many (or manybe not ANY) terrorists out there to begin with!

Additionally, terrorism is also a crime of opportunity. There are MILLIONS of targets in the US that could be ripe for even a lone gunman willing to die. Churches, Wal-Marts, sporting events, tens of thousands of little yellow school buses full of children each morning...

Why spend 6 billion a year and stripe search everyone when locked cockpit doors and pax that are more aware will prevent hijackings & turning planes into airborne missiles again?