The question could be asked: if one was unaware of one’s own nonexistence before their life began, why then should they fear nonexistence after their life has ended? What is there to fear in death if there is nothing to be aware of? One may claim that the nothingness after life will be no more painful than the nothingness that ‘was’ before life.
However, this line of thought assumes that one has nothing to lose by succumbing.
It assumes also that nothing has been gained in life – that whatever life has been experienced has no more value than the nothingness that was before them (and perhaps some beings do hold this to be true of their lives).
Certainly, one will not be aware of the loss of their own life, but is that in and of itself not a loss? It will not be actively painful, certainly, but there are far more horrible things in the world than pain. Nonexistence is one of these things.