The thing is that aging doesnt bring any benefits so its paradoxical to natural selection, or at least it seems to be the case.
Well, the point is that everything's going to die anyway.
There's a story about Henry Ford. It seems he told his employees to look through junkyards at junked Model Ts and report on why they were broken. When they came back, he observed in their reports that there was one part that was
never broken, no matter how beat-up the rest of the car was. So Ford went to his engineers and said: "You're using too much steel in that part, make it weaker."
If you're going to die of starvation, disease, or being eaten by a sabre-toothed tiger
anyway, then there's no point in natural selection making you immune to aging, because a variation which would make you resistant to old age
after you're dead has no selective advantage. And a variation which makes you more likely to die after you're dead is selectively neutral. And such a variation which also makes you fitter and healthier while you're alive would be
favored by natural selection.