I think part of it might be that evolution only cares about how successful you are in passing on your genes. Why do salmon die after mating (its due to exhaustion)? There isn't really any reason they have too, but they successfully passed on there genes so there is little evolutionary pressure on what happens afterward.

I think it is the same for aging. There isn't really a reason for aging, it just happens because there is less evolutionary pressure once humans pass the age when they normally procreate.

Of course I don't have any qualifications and this is just my hypothesis so take it as you will.