Except that could encourage people to stay out of the right lanes -- leading to confusion or wasted lanes when trying to pass people, which happens all the time. If you closely watch a german highway, there are 2 to 3 lanes (per direction) for the most part, and the speeds are quite consistent: The rightmost lane is full of trucks going 60 mph (90-100 km/h), while the middle lane is going around 80 mph (130 km/h), which is the advised speed, or Richtgeschwindigkeit. The leftmost lane is left exclusively for passing slower cars and those going 100+ mph (160+ km/h).