This may be something I'm making up, but I have the feeling that the fame = immortality concept came out of legacy: people wanting to create a family that continues on after themselves (and is rich, powerful, etc). Which makes sense, because then we're talking about a logical extension of the reproductive instinct. But in the modern world even that seems unreachable to me: we're so utterly different from our grandparents that we might as well be aliens, and the same will probably hold true for our own grandchildren.
I guess all that puts me in the Mike Tyson school of thought on legacy: "We're just dead. We're dust. We're absolutely nothing."
Another is that which is kind of intangible and describes the person(again not personal details but what they think about the self and ideas), like an autobiography, but still is very hard to get at: it's like they say Being in someones shoes.
It's impossible to understand both of the above kinds of thoughts, in general, because conscious thought is utterly temporary and highly subjective. And more so for the second kind of thought above for most people it is true that, their complex self is meaningless to others.
It's likely why you mentioned you feel disassociated from what your parents/grandparents thought.
> we remember factoids... I might remember 7 things about Teddy Roosevelt... but those things do nothing to represent the complex individual he actually was.
I've thought this before when looking at Wikipedia pages. Especially for less famous people with thin pages, they'll cite just a handful of news articles or press releases in which the person appeared. If there were a page like that for me, or the people that I know best, the collection of factoids would be a laughably inaccurate reflection of who we really are. Someone told me that it's important to write an autobiography for this reason.