There's no mobile phone camera that's even close to GoPro's video quality. I can't put a phone on my helmet and go skiing or windsurfing, it will simply not survive.
There's also the issue of wind noise. GoPro hasn't completely solved wind noise, but they are leagues ahead of what a cell phone is going to achieve at 65mph.
I would call "windsurfing" and "skiing" extreme cases of POV usage. The normal application of POV cams could easily be filled by a more practical mobile device such as a cell phone.
Why is a multi-function device without any real buttons or durability a more practical mobile device? I sure wouldn't want to put my phone on the outside of my car or on my quadcopter or try and use its capacitive touch screen underwater.