Some of us do care. I've been more open about where I am on this board than any other, but I don't usually like mixing my online activities with my offline world. There are online places where I wouldn't want to be recognized offline. (such as when I'm discussing my job hunt or dieting, for example - I wouldn't want anyone thinking I frequent websites/forums I'd be embarrassed to own up to!)
For others, it's just a privacy thing, being careful. Nothing wrong with that.
Btw, Marsye, I love your avatar picture!