Because if you could command more, you would demand it. And if the buyer/employer believed you, you’d get it.
Having been both an entrepreneur and a corporate guy, I find the notion of somebody being underpaid a laughable gripe.
“Those clowns are completely underpaying me! I could get twice as much somewhere else!” is a common lament among many.
My response to them is “Start up your own shop to sell the same service and see what people are willing to pay. Then you’ll know exactly what you’re really worth.”
(Hint…actually a lot more but so few are willing to take the chance that those who do make out like rock stars.)