not everyone is as aware of their capabilities as you are. Some people believe they are invincible, that they are in control, in charge, and can multitask at 70 mph without a problem.
If an accident happens, then it's always, always, the other guy's fault, never theirs. But the link between walk/talk/chew gum and driving/talking/phoning is specious and thin at best.
Few people walk in heavy traffic on freeways, chewing gum. Few people walk at 60 or 70 mph. But many people fail to realize that anything, ANYTHING, that distracts the driver from his main job of driving, can kill him, or someone else.
There are plenty of competent drivers out there who simply should not use a cell phone. It has nothing to do with driving ability, it has to do with paying attention to the road. Here's a silly analogy that you might want to consider, IronMan; how would you feel if you looked up from the operating table and saw that your surgeon had a cell phone? "I can multitask" he says, cheerfully, as you drift off...