Perhaps people emailing ten years ago were more adaptable to new technologies. More of the people who started emailing in the last ten years are likely to see emails as a form of traditional letters.

I don't see that as true- even tech savvy people seem to do it. I think it might have something to do with email sometimes being seen as a more formal type of online communication, as opposed to instant messaging, or even perhaps less formal emails (there's a distinct difference, in my mind at least, between formal and informal emails; formal emails are written much like letters, whereas informal emails are more like instant messages or Facebook comments).

Younger people still report to older people, and usually if you're emailing someone who is your superior (your boss, a customer) you let them determine how formal things will be. This doesn't hold true in every case, but it's common enough to explain the trend.

I sign email for the same reason I say "hello" to people when I see them on the street--doing so fulfills social expectations regarding kindness and politeness, making communication seem more personal.

If you sign your email "Thanks, Brian" or "Regards, Brian" or "Cheers, Brian" then that is much better than "- Brian" or nothing at all. Similarly, I include "Dear XXX," or "Hi XXXX, " or at least "XXXX," at the beginning of the message.