I guess it's all a trade off of material/weight vs performance. If you make a more rigid passenger compartment then it might weigh more and your car might be slower. Or maybe you have to spend more on a stronger type of steel that is more expensive. Or you have to make something more complicated to be safer.
So on average maybe the US cars come down on the lighter/cheaper/faster side of the curve. Obviously the trade offs are incredibly complicated.
Crash survival depends on the dissipation of kinetic energy into the structures of the vehicle rather than the structures of the human body. Even in well-designed vehicles, more kinetic energy means more danger.
Quite simply US car manufacturers don't have to make their cars as safe, as a result they include bare minimum safety requirements to pass regulation and save on costs. As another poster says it also seems that they do little R&D in this area either, again probably to save costs and to be able spend money designing more inefficient trucks for single occupants.
That's only part of it, US law requires safty systems to designed around for an adult not using a belt which results in a less safe car when someone wares a belt or is not an adult male.
PS: EU microcar are terrable in high speed crashes, but there not designed for high speed driving either. So it's generally not a real world issue.
Well thats some dumb regulation right there. There is a law that requires people to wear seatbelts and then you make another law that says design safety equipment as if people are ignoring the original law.
This penalises law abiding citizens to protect those who decide to ignore the law and not wear a seatbelt.
I meant: what causes the difference? Why is there such a big difference?