It is no wonder that the US tax rules get more complicated every year when the most interested parties, accounting firms and the IRS, benefit from convoluted tax procedures.
Consider a proposed bill to change the tax code. The change complicates things so that more people and businesses will need accountants or tax services those companies will lobby and support the bill. Since the marginal change (% of profit) to the befitting companies is larger than it is to the companies that pay the tax law will trend toward complexity.
Said in systems theory vernacular... The feedback that increases the complexity of tax code is greater than the feedback reducing complexity. This causes the overall system to increase in complexity. It's a vicious cycle.