What do you live for?

[Pyrrho]

... Why have you not killed yourself yet?

I think you have the matter backwards.  One does not need to decide to keep living.  If one makes no decision, one keeps living (excepting for accidents and such things that are not acts of will).  One's inertia is to keep living, not to kill oneself.

This is discussed in David Hume's essay "Of Suicide.":

http://oll.libertyfund.org/titles/704#lf0059_label_889

So, in order to kill oneself (willfully, not accidentally), one must decide to kill oneself.  But one does not have to make a decision about living or dying to keep living.

So the answer to your question for everyone who is alive is either that they have not decided to kill themselves or they tried and failed at it or they have just recently decided to kill themselves and have not done it yet.

One of the really interesting things about this is that people who live horrible lives often do not kill themselves.  This is because animal species (including humans) have a natural aversion to suicide (otherwise, they would not live to produce the next generation).  Fundamentally, the "choice" to live is not rational at all; it is the default position which it takes great effort to overcome.  Losty mentioned this natural tendency already.