You buy silver flatware instead of stainless steel flatware and you pay the higher tax. You buy a yacht instead of a cabin cruiser and the same thing applies.
You buy the "expensive stuff" so it's easy to see why you don't want a flat tax 'cause you'll finally have pay the taxes you truly owe instead of having your lawyers find loopholes for you so you can evade taxation.
There's no question that the tax code needs to be completely overhauled and simplified.
But there's absolutely nothing wrong with seeking to minimize your tax burden as long as you stay within the law as it exists.
If your situation is so complex that you need accountants and lawyers to figure it out, then you have to use accountants and lawyers.
But to use the argument that someone is "evading" taxation by exploiting "loopholes" and not paying what they "truly owe" is a standard liberal class warfare tactic.