Why do we fret about security?

Stephen Walt opens:

The United States has the world's largest economy (so far), and the world's most powerful conventional military forces. It spends about as much on national security than the rest of the world combined, and nearly nine times more than the No. 2 power (China). It has several thousand operational nuclear weapons, each substantially more powerful than the bombs that destroyed Hiroshima and Nagasaki in 1945. America is further protected from conventional military attack by two enormous oceanic moats, there no great powers in the Western hemisphere, and it hasn’t been invaded since the War of 1812.

[..] 9/11 reminded us American security is not absolute, of course, and the strategic advantages I just outlined are no defense against climate change, pandemic disease, or financial collapse. But surely the United States is about as secure as any great power in modern history. Yet Americans continue to fret about national security, continue to spend far more on national security than any other country does, and continue to believe that our way of life will be imperiled if we do not confront an array of much weaker foes on virtually every continent.

He goes on to explain threat-mongering.