Because they can't bear to make vehicles smaller. That's the real terror on American roads. Something like 60% of vehicles are huge because it makes the driver feel better. And indeed, it makes the driver safer in a collision - at the cost of making everyone else in the area less safe. Something has to be done to stop this trend first and foremost.

After that, when vehicle sizes start trending down towards reasonable, we can look at getting speeds up without reducing overall safety.