The important part is not the physics, fundamentally this is a statistics problem across some population. "heavier cars are safer than lighter cars in equal-mass collisions"...right, but that also means the heavier your car, the less cars you'll encounter on the road that are heavier than you are. The person in a 90th percentile weight vehicle drives in a world where they are on the better side of a head-on collision 90% of the time. And because of that, you can't transplant cars from a vastly different weight distribution population and expect the same safety results for them.
The Autobahn does put speed limits on larger vehicles like buses and trucks, to try and limit the worst of the high mass + high velocity combinations possible. That's far easier to do than something like parallel roadways.
It's also worth noting that most of the traffic on the specific chunk of US highway I referenced (I-95) has roughly the same car fatality rate as Germany. There's a handy chart comparing Autobahn safety that breaks things down per-state in the US. The best US entries on that list overlap heavily with the busy parts of I-95. Delaware, Maryland, Maine, Massachusetts, New York, Virginia, New Jersey, New Hampshire, those are all states where I-95 is the primary north/south motorway. Those also happen to be some of the richest states in the country, meaning people are buying higher quality cars too--which may also be the case for typical Autobahn traffic. There are a lot of things that correlate with highway safety in some way.