https://www.theringer.com/nfl/2023/7/18/23798560/nfl-running-back-market-saquon-barkley-josh-jacobs-franchise-tag
Running backs had already lost the positional value war; when Saquon Barkley and Josh Jacobs didn’t get new deals, the NFL’s best ball carriers realized it
This is a microcosm of how employment works in the world at large.
You aren't paid based upon how difficult your job is, nor are you paid strictly based on how much value you add. You're paid based on a function of value added, AND how replaceable you are. Essentially supply vs demand. If your job is hard and you add a ton of value, but you're easily replaceable, then you won't make much money. There's just too much supply. It doesn't matter that RBs are important if you can just throw a rock and hit someone who can fill the role.
Likewise if you're difficult to replace, but don't add a ton of value you also won't make a lot of money. My best guess for an example of this would be long snappers.