The big problem is the short shelf life of RBs. The average younger RB in the NFL is better than the average older RB in the NFL. As consequence of the position, they take a beating, and that beating shortens their careers.
When it comes to working with the cap, the value for almost every RB in the league isn’t there. There aren’t too many Adrian Petersons in the league. No matter how good these guys are when they sign, teams know that, in two to three years, that value will disappear.
Fans were calling this out when Ezekiel Elliott got his big contract, and they were proven right in that case.
That’s definitely some of it. But backs can get better (or at least more prominent) through their careers. Thinking Marlon mack, pretty much every 9er RB, even Henry. Reading through the list of older RBs is a shocking list of former greats though. So many who used to be stars and now can’t do more than bit parts.
But also replaceability. It’s so easy to find running backs who can be effective cheaply either in the draft or free agency. The latter is self perpetuating and related to the above, teams shipping out good older guys because they don’t want to commit to long deals with older guys. I suspect the former has to do with how many of the best athletes play RB at lower levels so lots of pro guys know how to do it.
Then there’s positional value: the importance of running has diminished and you only need 1RB on the field compared to say 3WR typically or 5 oline.
I think one massive problem is just how long rookie contracts are. By the time they’re done with their fourth (or fifth) year, most of them are at or past their prime. Meaning they’ll degrade after that.
Rookie contracts in general should only be two years without a third year option. Anything longer just serves to take advantage of relatively young players when they’re at their healthiest (health being the biggest hindrance for RBs).
