I’m guessing it’s too long to fit on the bed and you’d need to leave the tailgate open but perhaps they didn’t have straps to tie it down with or maybe they just live really close and didn’t want to bother. Also the fact that they bought a single board indicates that this is probably just some average joe with no knowledge about construction or transporting the materials. Obviously they know they’re in a truck so there must be a reason they’re not using the bed.