I think people find ways to mentally avoid suicide, even if they really think it is the only option. For me? I didn't want to leave a mess. Seriously. That was my thought process. At one point I was counting pills and pacing for hours but changed my mind because I didn't want my family to find a mess (vomit, blood, etc.). The only other time I seriously considered it I was on a business trip and the mess part was not an issue but then I worried my parents would forget to feed my cat. Two sort of random reasons not to kill myself, but I think it was my brain saying "back up" and "do not do that."
I said to my therapist once that the only thing I think I have ever truly failed at was killing myself. I seem to always land on my feet through sheer stubbornness, but that was something I could never make myself do even if I really wanted to.