aging mostly happens after an organism reproduces, so there is no evolutionary pressure to maintain fitness. there is no pressure to develop maintenance mechanisms...
Why is that supposed to be a problem? Plenty of species will happily eat their own young.
Humans seem to reliably be able to handle environmental changes expected in their life spans. Other species not, and I dare say most other fauna has much a faster generation iteration rate because they are much more sensitive to the environment. Ergo evolution made their lifespans shorter so they can iterate faster and respond with a tighter gene fit to the environment.
Humans have their fitness less correlated by genes so a longer life span is allowed, but not boundless. In a sense every body must die so that new solutions/iterations can be ran.
Note: This is completely outside my expertise, so make of that what you will.