Same logic:
We all get old and die, shouldn't I just die now?

Answer:
Many people smarter than you and I have asked "to be or not to be"... It's an age old question. For the most part people enjoy existing. It's far more interesting than death. And even if you dont, death doesn't make you enjoy it any more. It's solving a temporary problem with a permanent problem.

You're the kind of person that realizes that things can REALLY happen, which can REALLY end it for all of us. But rest assured, it isn't because of some made up date like 2000 or 2012. Smart people are thinking up the real reasons before they happen. Stuff like Yosemite, Ice Caps, Asteroids, Nuclear Winter, Nanobots, Viruses... Pretty much anything that REALLY poses a threat, we are actively trying to solve. We are better prepared than life has ever been in the entire history of the cosmos. It's almost exciting to see what happens when we finally look danger in the face, don't you think?

Anyway, our fight or flight thing is kind of meant for more primitive thought, where danger would have to stare us in the face to have meaning... Not anymore. We can think up such vivid scenarios that our subconscious is tricked into acting it out as if it's real. Just remember- save the fear for real, tangible things. No need going through pain when in reality nothing is happening. If **** goes down on 2012, then get scared, dont get preemptively scared, that's not what you were made to do.