I don't think "just" is appropriate. How many smart phones can handle rain or snow? How many support both GPS and GLONASS? How many have the UI (both screen and buttons) which fit with the needs of what many cyclists want?
Other than battery life, some of the reasons people prefer a dedicated unit are: better aerodynamics than a smartphone+case, support for ANT+ sensors, more rugged in the face of falls (even for a smartphone in case), more easily controlled by hands in a bicycle glove, and the ability to still have phone service even if the GPS battery is exhausted.
But that's just me being a Luddite. For now I think I'll stick to my phone's GPS if I need a map.
My riding partner has difficulty with converting the absolute direction of maps into relative left/right terms, and prefers GPS turn-by-turn guidance.
I go on cycle camping trips. I get lost most often when I go through cities, because I find it hard to keep attention on the map, pay attention to the traffic, and take in what I'm cycling through, all at the same time. I am tempted by a turn-by-turn GPS so I don't, for the n-th time, have to stop and redetermine my position and bearing.
That is not an adventure.
I have been "lost", in the sense of not being where I thought I was, and being further off the route of where I planned to go, many times. So far none of them have resulted in an adventure.
Nor have none of them been of the sort described in this article, where getting lost near sunset increased the risk of hypothermia.
What has resulted in an adventure is looking at a different route, wondering "where does this go?", and taking it - knowing that I have enough mapping technology with me to both see that it might be an interesting route, and not get more than temporarily lost.
Every time I move to a new city, one of the first things I do is go out with nothing but a bike, my house keys, and a little cash (just in case). Friends are still amazed I can get from one place to another so quickly without resorting to GMaps or similar.