Because for an individual, I don't view the value of a dollar as linear as income goes up.  Obviously the nominal face value remains the same, but the economic value (i.e. benefit) of an additional dollar to a person decreases as they make more of it.  $10,000 to someone who has $5,000 is a huge benefit, but $10,000 to someone who has $1,000,000 is much less so.  Similarly, taking $2,000 from someone who makes $20,000 is much more of a hardship (takes away more economic value) than taking away $200,000 from someone who makes $2,000,000.