Because those vehicles can move in the third dimension as easily as in the other two? Because this wouldn't require an intense amount of coordination between vehicles, and tons and tons of added security measures and protocols, given that cars can now literally fall from the sky and onto each other? Because the resources to build those magic cars are easily and sustainably acquirable? Because this does not need a much more involved inspection and maintenance infrastructure? Because energy considerations has been solved, and things "like cheap nuclear power" are safely and readily available? And the insane amounts of power required are so easy to distribute among those ubiquitous cars?

I just don't see how this is "efficient".