Unlike SUVs, pickups don't have an adjustable suspension (pneumatic or hydraulic springs) so if you had a low bed on an empty pickup, it would have likely compressed the shocks completely under a heavy load, or, alternatively, you'd had shocks so stiff that they would not move without load at all. Most people who carry heavy loads in their trucks prefer neither and opt for the "lifted" look due to the long shocks, which work both empty and loaded.