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.
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).