Health is part of it. RB's take enormous punishment, so it's very risky to invest a lot in one, but
You're right that it's more about how replaceable they are - how small the gap is between most of them. It's also become a bit of a self-fulfilling prophecy, as players who would have been RB's 20 years ago now don't want to grow up to play RB, so they play WR, DB, or even LB. So, there are fewer of those outlier talents that might be worth paying.
But, think of Breece Hall. He hasn't fully come back from his injury. He doesn't have the speed and burst he did prior to the injury, but he's still one of the best backs in the league. Give him bad QB and OL play, and outside a couple plays, he suddenly becomes useless.
You can say something similar about WR. The difference is that a WR can elevate QB play/maximize the passing game, and if you don't have great WR play, you need a lot of other exceptional things to go your way to overcome it.