Speculating - older individuals take resources and even if they were continuing to reproduce advancement comes through successive generations of genetic mixing. It's probably advantageous for the species if an individual has a child (mixing up genetic code) and then that child has children while the parent dies.
Otherwise you might have less biodiversity which may make the species less adaptable overtime (and maybe lose to another species with aging).
Otherwise you might have less biodiversity which may make the species less adaptable overtime (and maybe lose to another species with aging).