I have noticed most people have their email addresses publicly displayed (in some human readable form), or have web pages with ways to contact them. In that note, I have been contacted via HN before and have met some people as well. Considering the ultimate purpose of a Private Messaging feature is to allow communication between two willing parties - the existing infrastructure is already there.

Moreover, more influential people who very strictly value their time and privacy (Matt Cutts for instance) might not be comfortable having an open avenue by which anyone can contact them.

More importantly, if you have someone's full name, there's a reasonably high chance you can find a proper way to contact them - provided you are determined enough to do it.