Even with a concise and simple tax code the problem is that those with large amounts of wealth are better off spending a portion of that wealth on intentionally obscuring their assets and incomes than by simply paying their fair tax.

From the article:

> They often have trusts, foundations, limited liability companies, complex partnerships and overseas operations, all woven together to lower their tax bills.

Which is to say that...even if you set out a flat tax rate of X% it is possible to arrange situations where multiple trusts, companies, partnerships, etc, are arranged in such a way that even though they are operated for the benefit of an individual that individual has little to no real obligation via their engagement, and the interactions of those independent entities behave in such a way as to - on paper - present a reduced income then it becomes exceptionally hard to determine who owes what without devoting massive amounts of effort collecting all of these loosely related entities and analyzing flows of money.