Why aren't there a lot of canopies on trucks out there?

[JBV]

my bike (xl) i lay down right in the box, pull to one side. second bike, front wheel off, slide it down the side and secure it to the side of the cap with a bungy cord. works well most the time. if i were to be packing the back with bikes and lots of gear for a trip, or going on long rough road, i would take off front wheels on both, fork mounts, rear wheel blocked, etc, to secure them better.