There doesn't need to be an evolutionary advantage for everything (e.g. vestigal features) - this "uncertainty-blindness" could have just not been exposed to evolutionary pressure.
Very few 70+ year olds are reproducing so evolutionary advantage doesn't make sense in that age group as they have already reproduced (or not) by then.
Historically the aged sat around the fire and taught the young. Which encouraged their genes' propagation. Since Humans became social, its not been all about reproduction.
this is all extremely speculative, of course! but i would say that anything that affects your thinking can pose an evolutionary advantage or disadvantage. (to also answer a peer comment here:) including when you are past your reproductive age, since human beings are social beings, and our evolution cannot be regarded purely individualistically. to give an extremely stupid, but i hope illustrative example, a society where the old people go extremely stupid and start killing people around them is going to have a hard time ;)