I don't know too much about the USA side of trucking but in the EU the cab over trucks have been able to steadily improve their aerodynamics, and there has been a lot of effort into improving this. Obviously there is more frontal area compared to a passenger car but comparatively modern EU trucks are quite aerodynamic. (even though they may not look it)

There are other things to consider as well when building a truck, as mentioned the maximum length limits, comfort, power price and more.