Because they can often be found in the 2nd or latter rounds. Why spend a 1st on a RB, when you can draft a DE, DT, CB, TE, LT or QB in the 1st?
Of course there are those can't miss RB classes like the 05 draft...
AP is the exception...and even he fell in the draft due to injury concerns
I know the thread is getting long so let me repeat this for those who keep making misguided claims...
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To further my point on 1st round RBs lets look at some of the Starting RBs taken in the 1st versus the claim you can get them anywhere in the draft. As I made the list the first thing that was obvious was that there hasn't been a good RB draft since 2008. The second thing I want you to notice is that I will list every 1st round pick drafted, BUT I wont list the other
20+ RBs that are drafted each year. So even if you think the non 1st round RBs are better than the 1st round picks (I don't), what you have to keep in mind is that although you may find 2-3 non 1st rounders that are good look how many picks are wasted to find them. If you are drafting 25 RB's every year of course some of them will be found in a late round but for the most part the 1st rounders are the ones that have talent and end up being a feature back for their team.
Starting RBs last decade:
2012
1st: Trent Richardson, Doug Martin, David Wilson
Non 1st: Vick Ballard, Alfred Morris
2011
1st: Mark Ingram
Non: Mikel Leshoure, Demarco Murray, Stevan Ridley, Alex Green
2010
1st: C.J. Spiller, Ryan Mathews, Jahvid Best,
non: Montario Hardesty, Jonathan Dwyer, James Stark
2009
1st: Knowshon Moreno, Beanie Wells, Donald Brown
non: Lesean Mccoy, Shonn Green
2008
1st: Darren McFadden, Chris "CJ2K" Johnson, Rashard Mendenhall, Felix Jones
non: Matt Forte, Ray Rice, Jammal Charles, Peyton Hillis
2007
1st: Adrian Peterson, Marshawn Lynch
non: Michael Bush, Ahmad Bradshaw, Garrett Wolfe (LOL)
2006
1st: Reggie Bush, DeAngelo Williams, Joseph Addai, Laurence Maroney
non: Maurice Jones-Drew, LenDale White
2005
1st: Ronnie Brown, Cedric Benson (BOOoooo), Cadilllac Williams
non: Frank Gore, Marion Barber
2004
1st: Steven Jackson, Chris Perry, Kevin Jones
non: Michael Turner, Tatum Bell, Julius Jones
2003
1st: Willis McGahee, Larry Johnson
non: Justin Fargas, Domanick Davis, Brock Forsey (LOL)
*** That 2008 Draft was awesome for RBs***