I thought exactly the same!!
That's the reason I stopped using my iPhone and bought a Garmin Zumo.
The problem with using a phone as a sat nag is that they are designed to do many things to an acceptable level, but nothing brilliantly. A dedicated sat nag is designed for that and therefore focuses on just that.
As far as the iPhone point - they are not designed for lots of vibration as the level of shake you get on handlebars is not within the 'norm' range. It does not help that most clamps have no vibration damping to prevent the issue arising (although both my bike sat nag mounts do to be fair but they are designed for that).