SF could have a devastating earthquake at any moment and people are still there.
Down in LA someone is trying to build a skyscraper on an active fault line: http://www.latimes.com/news/local/la-me-hollywood-fault-mill...
A good portion of Manhattan will be under water if sea levels rise as well.
Then we all know what happened to New Orleans.
People seem to enjoy evacuating after the fact, not before.