Why is so much emphasis put on RB in fantasy football?

It's not that RB's score more total points than QB's. You're right about that. The reason RB's are generally taken first is the difference in points between them. The point margins of the starting QB's in the league are closer than those of the starting RB's. The varying degree of points between all-star RB's and the other starters is why they're taken before QB's.