The reasons are quite simple.  

1) Rich people hire tax attorneys, CPAs, and MBAs to ensure they pay as little tax as possible.  (See Trump, Rockefeller, Pelosi, Kerry [and his wife, nee Heinz])  or any of the millionaires currently in Congress.  

They pay each of these thousands of folks salaries in the six digits and above to meet that goal.  Failure to meet the goal means that all those tax attorneys, CPAs and MBAs are looking for new jobs to cover their Lexus' payments, their country club memberships and the private schools for their kids.

2.  Rich people donate to both Democratic and Republican Senators and Congresscritters' election campaigns and PACs to ensure that tax policies are put into law to their benefit.  (See Trump, Rockefeller, etc. and the millionaires currently in Congress.)  

See Hillary's list of donations over the years as an example for Presidential candidates.  Trump even donated $110,000 to her 'Foundation'.