One way of looking at evolution is to see yourself, the individual, as little more than a disposable short-term delivery system. The focus of evolution is propagation of the germline, and aging exists in its present unpleasant form because in 99.9% of all complex species there is no selection benefit in avoiding it. On the one hand, nature is red in tooth and claw, and the only system that survives in the wild is one that gets the job of replication done before a violent or diseased death. On the other hand, systems optimized for early life tend to fall apart and consume themselves in later life. The mammalian adaptive immune system is a good example, a limited capacity system that will eventually malfunction due to encountering and attempting to remember too many different pathogens regardless of all of the other issues of aging. Evolution led to that system because it works well enough to get by in early life, and because there is little selection pressure to avoid the inevitable crash later in life, when the chances of reproductive success are low.
Lastly, there appears to be a race to the bottom between long-lived and short-lived