Maybe its because all the parents are "friends" and not some totalitarian regime that they wish to avoid?
Technology, as mentioned, also helps.. go lock yourself in your room and live your virtual life without the hassles of actually being "independent".
AS the link mentioned above, AW has covered this pretty well too.
What is a potential interesting corralation is that the Graduated license is often only for the 16-18 crowd. once you are 19 or so, its a full regular license. So did the GL scare off folks and now we have an OLDER, less experienced first time driver who now can drive WITHOUT restriction. Insurance companies will let us know for sure
Money can be an issue.. but I am sure more parents would rather make a $150-200/mo lease payment on a new "safe" car for Johnnie or Suzie than make them earn the money to drive a 10-20 year old beater.
Then again, with the recent "google car" announcement, kids soon won't know that some people actually drove for
enjoyment