Really simple, same equates in a similarity to automotive fuel efficiency, drag really increases. 45-65MPH seems like a sweet zone, anything over the force of air increases exponentially as drag. To add a personal note, I found my fuel efficiency better in my motorhome (really high drag) at higher altitudes even though I was climbing and descending grades.
This is why airliners have been built to fly at altitudes up to 420, smaller pressure tubes in the exec-jet upwards of 50k as they can achieve a higher DP.