Because families are far flung? Because you can't send them photos over the phone? Because you can't contact all of them quickly and share so much easily? Remember, you can share as little or as much as you choose - I've got a younger brother on Facebook who won't let any of us see his pages (he's distancing himself, long family saga). No one can see my pages unless I allow them as a friend. It's not as strange as you think once you actually see it in action, then you decide how much or how little you want to use it.
I still see my mother several times a week, my sister, my neices and nephews - but we all can share photos amongst ourselves and generally yuck it up over them. That's all it is for me.