Listen to what actual OL have said. The problem with blocking for a running QB is your back is to them, and the defense can see them, so suppose RG3 drops back and takes off to the right, his right tackle doesn't know he's doing the left defensive end now has an advantage. A more stationary QB, you know where they are.
A running QB that is all over the place makes it difficult but that is not what guys like RG3/Kaep are. They are sitting in the pocket then escaping. By the time they get outside the tackles they are so athletic that no defensive lineman is going to catch them.