Why do truckers use every single gear when driving without a load?

As a driver of a car, I noticed that most truckers hauling flatbed semi without a load start from a stoplight using every single gear. The truck accelerates real gradually as if the trucker was hauling a speck of neutron star stuff because of this practice. Now, when hauling a load (especially one of max legal weight) the trucker has no choice. But with a flatbed with no load, a car driver would think that the trucker would skip a few gears to accelerate more quickly. Is it just habit, or is there a real reason?