There are also at least 20 runningbacks that come out each year that are capable of contributing to an NFL team. There are several WR's, DLmen, LB's, OLmen, etc... but only one and situationally two rb's on the field at a time. Furthermore, there are constantly guys on waivers or unsigned that can play at a high level. That is not nearly the case with other positions. Look at Denver, McGahee (who was dumped himself) goes down, and they plug in a guy in Moreno that nobody in Denver would have cared if he got traded in week one. They wouldn't even would have cared if they cut him. He was literally the fourth string RB at one point.
With the league being more passing oriented than ever, it simply doesn't make sense to focus your team's duckets on a position that is easiest to replace. That money needs to go to guys that play positions like QB, which is much more difficult to fill.