Because I kinda want to keep up with their big events. Like kids, marriages, jobs, a new house or whatever. And I see some of them occasionally when "back home" and then it's nice to have kept somewhat superficially in touch. And fb works for that. It's just all the day-to-day stuff no one posts anymore.
There's also the unspoken norm that un-friending someone is rude. I frequently prune my following lists and then I run into someone I've met and imagine them getting slightly upset that I have unfollowed them. Maybe that sounds silly, but I'm pretty sure that a lot of my peers would say similr things.
The way that we describe the action, "un-friending", probably influences the notion that it's a serious / rude thing to do. We need some kind of UI that emphasizes that it's not a big deal whatsoever and is not a rude thing.
