Its not that we dont matter at all per se. Its a bit hard to put it into words what Im trying to say. Ill flip the question onto you: because we matter, regardless of how little or much, would you be willing to live for 500 years? What about 1000? Its kind of like were bartering over the number of years its acceptable (usually socially, because as you can see from this thread as soon as someone sais "actually i dont want a long life", others tend to not be able to comprehend why that would be) to want to live. My point is that mattering doesnt matter to me. We think we matter, but most people dont really. Most people dont even know or remember their great grandparents names. Thats just 2 generations removed, and unless they achieved something remotely special, they no longer exist.
Part of it of course is because of our deeply buried subconscious need to live, or rather avoid death, because ultimately we are animals and most of what we do just boils down to that. Thats the reason food tastes good - because if it didnt release happy chemicals in your brain, you would be less motivated to seek it out, and the species would not survive (same reason sex feels good btw, same reason most things feel good or bad). Once we evolved a modicum of intelligence beyond other creatures, this paralysing terror of the unknown of death spawned religion, early on as proto-religions and in more modern times as the religions we are familiar with today, but the long and short of it is that humans are hardwired to be terrified of death (at least their own) and attempt to prolong their life as much as possible - whether in this life or with some concept of an afterlife. Its the same reason without really understanding the mechanics behind your decision, you feel like you want to live longer, and the same reason I havent yet done a long jump with a short sudden stop. Its very difficult to reason yourself past your animalistic evolutionary instincts that have kept you and literally every single direct ancestor of yours alive, unless you have some kind of illness (of which I believe people who commit suicide suffer - its not really a choice made with a healthy mind, and we should be treating it more like an illness IMO so that we can help these people)