...most fantasy characters I read about don't ever mention their parents. Not even Aragorn that I recall, and we have his full family tree available!
It's a long time since I read "Lord of the Rings", but I seem to recall him being referred to as "Aragorn, son of Arathorn" quite a bit. Plus, in the extended edition of the "Fellowship of the Ring" movie, Aragorn and Elrond have a discussion about Aragorn's mother at the grave of said mother.
Plus, of course, Aragorn's family history informs who he is to a significant degree. It establishes his right to the throne, of course, but also gives rise to his doubts as to his fitness to rule.
Why bother when that stuff won't come up in campaign?
If the stuff
won't come up in the campaign, then the only reasons to do it are if it will enable you to portray the character better, or if you enjoy doing the work for it's own sake. Otherwise, it's the equivalent of the DM doing a whole load of world-building that the players will never see.
However, most good DMs, in my experience, will go some way towards bringing at least some of the character's background into the campaign in some form. They probably won't use it all, but may well use key parts. In any event, if the player does that work, then it might come into the campaign; if he does not, then the character's background
cannot come into the campaign.
I think a lot of it stems from the Law of Conservation of Detail. Many players don't want to "make NPCs" (which is what family members usually are, if they're not PCs) or want to only make "useful NPCs" (like Shadowrun contacts).
I think there's a lot of truth in this. To a certain extent, I think it goes to the question, "whose campaign is it?". If it is the DM's campaign that the PCs are just passing through, then all the world-building, the NPCs, and so forth are really under the remit of the DM.
However, if the campaign belongs to the group as a whole, then why shouldn't players be empowered to add elements to it? Why not add a family? Heck, why not detail an entire home village?
And then a lot of DMs want a lot of detail. I know it won't come up in campaign (as I don't ever see that stuff used against me) so why make me waste time on all this detail?
That's fair enough. I used to think that by having a detailed process for fleshing out a character's history, personality, or background, I got a better campaign. In truth, I just got the same variations on a theme over and over, followed by the players essentially 'playing themselves'.
I now have players detail only as much as they want. I then may or may not use it. That said, I'm now leaning back towards having a slightly more detailed approach, as things seem to be getting very 'samey'.
And if it does, well, family is an obligation.
Ah, but they don't have to be. Perhaps the family provides a conventient home-base for the party. Perhaps they can shelter them from enemies when no-one else will. Perhaps they can provide an unexpected and useful contact to the PC. ("My cousin Balin will give us a royal welcome...")
Of course, that relies on having a DM who will allow the player's background to come into play in this manner.