A combo of 3 things:
1.) A lot of RB success is dependent on players around him.
2.) It's a lot easier to find a competent RB than at most other positions
3.) Salary cap
Some RBs are certainly better than others. However, if you have a solid OLine and a good passing attack that keeps defenses from stacking the box, a "good" or "competent" RB can get you close to the production that a "great" RB.
Of course, you'd rather have "great" as opposed to "good" or "competent". But this is where the salary cap kicks in. If there were no cap, pay the best players at every position as much as they want. But the NFL salary cap is very restrictive. Good teams tends to have good players at the premium/harder to replace positions like QB, Edge, CB, LT, etc, who are already probably making really good money. If you don't have a whole lot of cap space left, you have to find a way to get cheap production. So you get most of the production for a much smaller percentage of cap space.
And I think the Patriots are a good example of NOT paying RBs. Yes, they've gotten really good RB play. However, they've always had really good OLines and obviously great QB play. And it isn't like they've one really good RB who they payed a lot of money too. In the last 4 years, they've had Legarret Blount, Rex Burkhead, Sony Michel, Dion Lewis, James White and even Mike Gilleslee be contributors. none of them have made even 2mil in a season for the Pats. They've been able to find backs that good enough to produce for real cheap, and rather than pay a RB bigger money to keep him, move on and find another good back that's cheap.