Because of the law of diminishing marginal utility as applied to income [1]? Once you hit a certain level of income, other factors can start to outweigh the marginal utility of a higher salary.

For example, I could maybe make more money working in high frequency trading, but I don't want to. It wouldn't be a life-changing amount of money, and I value the ability to work on interesting problems in the open, with less concern for ethical issues†, over the potential additional income.

[1]: https://qz.com/1211957/how-much-money-do-people-need-to-be-h...

†I don't want to start an argument about whether HFT is unethical; in fact, I don't think it is necessarily unethical. I'm just saying it would always be at the back of my mind.

Professor Christensen says it better than I ever could: “Motivation factors include challenging work, recognition, responsibility, and personal growth. Feelings that you are making a meaningful contribution to work arise from intrinsic conditions of the work itself. Motivation is much less about external prodding or stimulation, and much more about what’s inside of you, and inside of your work.“