Somehow, despite not understanding why exactly we exist, what our purpose is, and that at any given moment we could die and cease to exist, we don't go insane. Do you have any idea how crazy that is?
I don't get why people need a reason to exist. To me it's clear that there isn't a reason and we don't have any predefined purpose, yet most people create their reason. Maybe they live for their loved ones, maybe they enjoy life. And regarding death: it's scary and doesn't happen very often, so it makes sense not to think about it too much.