Why not this??

Been Browsing this forum for awhile and my team mate and I are curious about hotshotting and expediting.
My question is this:
Why not a single axle hiway tractor with a 40' gooseneck? I realise it would be alot like a medium duty setup.
for example:
A volvo vn 630 single axle, with a deck on it, and a recessed ball hitch. this way one could move a small deck load OR hitch up the wagon and move bigger freight.
Find one with a detroit in it or volvo engine with around 450 hp. these engines get decent economy hauling 80,000 lbs. I bobtailed one and got 15mpg on a mountainous, gravel road. i also hauled a pup trailer on the same road and got 13 mpg. this was done with an IHC 9900 with a 500 cummins!
pulling a 40 footer and performing some fuel mpg improvements should keep the economy better than alot of the pick up and trailer set ups I've seen on here.
Any ideas or thoughts??