Others here have hit on the main points, and Richard Dawkins has made interesting comments on it in The Extended Phenotype. Here's my distilled answer: Death d

Others here have hit on the main points, and Richard Dawkins has made interesting comments on it in The Extended Phenotype. Here's my distilled answer: Death due to ageing may be dwarfed by death due to other causes. For example, look at short-lived mammals like rodents: An individual is likely to due of predation before senescence. And, everything has a cost: The (short-term) survival and reproduction cost you pay must be worth it.