Smart phones weren't designed for use while driving but neither were maps, kids, sleepiness, or being drunk.
Let's take those in turn, shall we?
Maps -- traditionally, most people only used these occasionally, relying on street signs, visual cues, and other general direction sense. For long trips, a lot of people would have a travel companion to use the map. And in any case, map use was usually NECESSARY to actually find your destination (which was your entire point of being in the car).
Kids -- can be horrible distractions. But again, like maps, you can't really avoid dealing with them.
Oh and most people don't really WANT to be distracted by kids and maps -- they're just forced to deal with them while driving. People WANT to look at their phones and thus seem drawn to them.
Sleepiness and being drunk -- the hallmark of bad drivers. The latter is explicitly illegal. The former is generally avoidable too, and if a cop catches you weaving and pulls you over to find you literally asleep at the wheel, you might end up with problems too.
Contrast this with phones -- which the present study says a lot of drivers use FREQUENTLY and which don't have the necessity element in most scenarios. Unlike maps (which actually get your where you're going) or kids (which can't be ignored), most of the non-mapping features of your phone use CAN be delayed. People just choose not to.
I'll finish up with " something something forest for trees."
Smartphone screens as distractions are a serious new threat. Like drunk driving and fatigue, they are not a necessary element to the driving experience and are easily avoidable... and yet people seem unable to stop using them even when doing what's pretty much the most dangerous activity most people do on a regular basis.
It's an addiction, more serious than drunk driving, because at least most people seem to recognize the dangers of drunkenness -- whereas lots of people seem to want to downplay the seriousness of the new smartphone distraction threat or, like you, just pretend "it's always been this way!" No, it hasn't. This is a new one which -- at a minimum -- ADDS to the previous potential distractions and compounds the danger.