The ground deforms. Wheels push ruts into it that stay for the whole run, so the line you cut on the way up is the line you meet again. Deep mud drags the truck down and sinks it, spinning wheels dig themselves in, and clods fly off the tyres and build up on the body.
Water drags and lifts, splashes and leaves a wake. Rocks and logs take the suspension through its whole travel. The truck is heavy and planted: it does not bounce off an obstacle, it climbs over it or it does not.
Ten seconds of fighting up a muddy ridge is the product. Getting stuck burns fuel, which is why tyres, the diff lock and knowing when to rock out of a hole matter more than the throttle.