I can see what you're saying, Dan. But it's a bit "absolute worse case scenario" to be fair, isn't it.
The standard cable isn't armoured it's full length, only a short mid section along the centre of the car so even running a new power cable through the body you'd have the same risks at either end.
In terms of the underside of the car getting a serious scrape, that would have to be some scrape! One that completely detaches the exhaust first, which sits much lower and is large enough to still be an obstacle that can damage the cable. I can't really envisage something that could cause that kind of damage that wouldn't also come with a huge list of other very serious risks anyway.
Of course if you have a very low car and the cable is the lowest part then yes, there is a large risk but in that case you also have a risk of your fuel lines being a problem too.
If the option is available then there's no reason not to run the cable through the body instead except the risk of another point of ingress for water, but how many cases do we hear of people tearing off power cables? I've never seen one on here or any other forum thus far.