Its a completely different type of running, and skill set. Rbs get low, run behind their pads and the best ones that last dont get hit square on a lot.
When a qb scrambles they are upright. You have to keep the ball live to pass as long as possible to stop the D from clamping down on you early. And that leads to running tall. You cant absorb nearly as much of the impact with your pads and pre loaded body.
A rb like harris is like a battering ram. Initiating contact causing the force of impact to be split with the defender, when you initiate and arent exploded from low hips behind your pads you take more of the blow.
On top of all that, qbs get hit un protected every couple of games at best. Qbs, also have to maintain healthy arm and body rotation. A RB can play and play well with mild shoulder and hand issues.
You see a lot of injuries with tall rbs that dont get low and power through the line. Qbs cant run like that. A rb can be a battering ram for a career. Qbs, cant.