Thats because Cam outweighs 90% of the defensive players trying to tackle him. When defenders see an avg QB start to scramble, their eyes get huge as they envision that huge bone jarring hit. When Cam starts to run, defensive players close their eyes and hope Cam decides to cut rather than run over them.
I'll say the same thing about Russell except he's a lot shorter than Cam. But the way he was helping his team block in yesterday's game, he looks like he can take some hits and keep moving.