From a biological standpoint, we die because we didn't ever as a species need to live that long, so it wouldn't make sense to invest any energy into keeping us alive any longer. When you raise as many children, who are themselves able to reproduce, you "fulfill your purpose", so that your genetical material spreads. There is no purpose to life, you are just a robot, a slave to your DNA. If genetical material wouldn't spread, there wouldn't be any life.