I don't know them so why should i care?
Absolutely ridiculous - yet it seems to be the normal human way.
Why are we like this?
I came one step closer today in understanding when stumbling across an idea called "The Monkeysphere" which is based on a correlation discovered by Primatologists decades ago. Apparently as Primatologists have noted, the size of a Primate Species neocortex is directly correlated with the number of individuals a Primate can form stable inter-personal relationships with.
Since this discovery another guy, Robin Dunbar, took this idea to the next level resulting in a framework that can accurately predict the number of individuals comprising a primate species' social group by analyzing that species’ neocortex size. For chimps, this number is around 50, while Humans (yes we are Primates) have a upper limit of around 150 individuals per social group.
What does this all mean and why is it important?
Back to the original question: “why are we like this?” One part of the answer seems to be that humans are geneticly predisposed to have a limit to the number of people we give a damn about.