Tipping those who set their own rates?

Hi everyone,

Please don't think I'm a total asshole for asking this- why is tipping necessary for someone who sets their own rates, i.e. a $750 makeup artist? I completely understand we are paying for her skill, her time, her travel, her products. But...that's why she sets her rates. I am genuinely curious, why do I need to tip on top of it? Is it an American thing?