Part of that crazy amount of time is that is that phones have become a way to avoid boredom. 93 percent of people 18 to 29 say they have used smart phones as a way to ward off a bad case of the bores (don't forget, boredom can be good).
That boredom response drives many to play games like Candy Crush or Angry Birds, once upon a time before it was made into a movie no one asked for. The stacking of small goals and the revelation of new levels causes the brain to release dopamine, creating what is called a compulsion loop. Most people know exactly what that is, having been unable to stop playing one game or another because there's a simple time-wasting pleasure in it all.