While not fun, I can think of a couple reasons. I'm not an expert in aerodynamics, but I'm fairly sure a truck of any type, or bricks, don't create much if any downforce and it's simply their heavy selves that keep them on the ground at silly speeds. Then there are the tires and the centripetal forces at that speed...doubt there are any engineered to hold the kind of weight a truck is capable of while simultaneously able to sustain the forces created at that kind of speed. That's the logic portion...the reality is lawyers. Fvcking lawyers killing your d@mn fun.
LOL