Highway engineers try to get a reasonable level of traction with a quiet ride. Motorists (the 4-wheel kind) want a smooth, quiet ride and have little thought for the coefficient of friction. After all, they have ABS and stability control to bail them out. Plus their vehicles won't fall over. And if the worst happens they are wrapped by an array of air bags. So the push is for quiet and smooth pavement. 2-wheelers are on their own.