The Importance of Defending Ourself

It's been over seven years since the last nuke was detonated in anger. The original episode was composed on the 71st anniversary of Nagasaki. Why are we still worried about nukes 70 years after their invention? It didn't take long to develop fighter aircraft, anti-aircraft guns and surface to air missiles. Then why, 70 plus years after Hiroshima, and 50 plus years after the development of ICBM, can we still not defend ourselves?