When you're in a high-enough tax bracket (in taxation regimes where such high tax-brackets exist), adding a marginal dollar to your net income becomes impractically expensive for the company, compared to basically anything else the company could offer you.

For some people, in some places, getting an extra $20k/yr tacked onto your net income, would cost your employer more than just, say, hiring you a well-paid full-time personal assistant, with the skills necessary to do enough of your job that you go from "overworked" to "takes off early most days to go play golf."