I think instead of flipping over, on their side, off a wall or another car in these games. The cars should just make the huge crunch, go from instant speed to a serious halt, perhaps just a little judder back and then forward again (or a jolt upwards, sideways etc). But not more than that. They should be stopped dead in there tracks, and for good. After that the entire bonnet should be scratched the hell and the steering/engine shot to hell. That's minus deformation. As is, both games react oddly, even when you factor in the fact that deformation might not be possible.
The main reason I'm guessing, is not because it can't be coded in or because manufacturers wont allow it. But because it makes the game less "accessible". To which my answer would be, make a different fucking mode for damage. Called "casual". And make the "simulation" damage, actually simulated. This goes for both GT5 and F3 mind. And all sim racers.