What is your definition of fairness?
It may sound fair to charge everyone 15% and call it a day, but think about this:
If Richie Rich makes 5 million a year but his lifestyle expenses only total 1 million a year, his after tax income is 3 m 250 th, more than three times his living expenses.
If peter poor makes 20,000$ a year (the higher end of the single person lower income tax bracket), his post tax income is 17,000$. Now his cost of living varies a lot. For a single person in Brownsville, Texas (a cheaper cost of living), it cost about 29,118$, For a single person in San Francisco it's 69,072$. Even for somewhere quite cheap, Tuscaloosa, Alabama, it's 35,779$.
So basically, Peter Poor is in the negative no matter where he lives. So if you're questioning fairness, who do you think is hurt more by the their tax bill, Peter or Richie?