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.