Many if not all of the studies of DRL's were done in areas where during certain
parts of the year the sun either does not shine at all or very little. Read "Far
North". In such areas you have to turn on your lights each time you drive the
car, its always night or close to night. If you forget....

While some areas of the US do have this problem most don't.

Vanagons sold in Canada did have DRL's and when used during the winter in such
far northern areas  were of course a great safety assist.

But most of us live and drive in areas more equatorial and have no such
problems.

Also, What about the driver who has low power DRL's but forgets to turn the full
lights on at night. Dangerous. If that same driver had no DRL;s it would be much
more obvious that his lights were off and that he should turn them on.