That is some mighty flawed analysis. Since Brees, Brady, and Rodgers weren't top picks then no NFL team should ever draft a QB number 1!!! Heck Brady was a 6th round pick, why pick a QB in the first 5?
McCoy fell because he went to Pitt. If he went to Texas or Bama he would have been a high pick.
Look at what most people consider the consensus top backs in the NFL and where they were drafted
Peterson - 1st
Lynch - 1st
Foster - Undrafted
Rice - 2nd
McCoy - 2nd
Doug Martin - 1st
Jamaal Charles - 3rd
Spiller - 1st
I could go on but you see a trend, while many top RBs slip through (same with top OL, QB, DL, etc.) in general you get your best players in the first.
As far as why RIchardson isn't getting too much of the ball or taking too many snaps the biggest reason is probably because unlike everyone else on the team he hasn't had all offseason and training camp to learn the plays. As the season progresses expect to see an upswing in his stats.
The percentage of starting RB's that were drafted out of the first round is probably much higher than the percentage of starting QB's taken outside the first round.