Ageing affects the fitness of an organism. Its physical and, often, mental skills diminish once aging has started, leading to increased mortality. Therefore, aging decreases the chances one has to see its genes passed on to the next generation. Aging is a bad thing . The main consequence of this hazardous lifestyle is aging.
Genetic drift can then lead to an accumulation of late-onset harmful genes that cause senescence.
Basically, although aging is, individually, a bad thing , there is no evolutionary pressure to spread longer longevity genes if no individuals reach an old age. Not as a direct product of evolution but as a phenotype that escaped natural selection. Senescence is regarded as a non-adaptive phenomenon. A phenomenon in which random events and other mechanisms eventually affect senescence as a by-product of evolution. In fact, there are so many exceptions that one wonders if they shouldn't be the general case and the general case the exception.