Because if he's making a below-market salary as it sounds like, he could be doing better. With a higher salary, he could have more savings, more security, more easily afford a family or the things he wants, donate more to charity, spend more on personal hobbies, etc. Money has many uses.

And I could ask you the same question: the startup founders and oil industry would live comfortably enough if they made a little less and paid him better; why is that not enough? Why should they be making more?