Besides those two, a third one is that the IRS has been starved on resources for a long time, mostly (but not exclusively) by GOP politicians who want to make i

Besides those two, a third one is that the IRS has been starved on resources for a long time, mostly (but not exclusively) by GOP politicians who want to make it hard to audit their rich benefactors. I had a friend who worked at the IRS his whole career until retiring (international tax law) who described the horrifically ancient systems they were forced to work with and the seriously budget limitations they had to live with. The IRS manages to miss enormous amounts of tax because they cannot afford to investigate the more complex situations, so in that aspect the politician's goals are successful. Yet investment in the IRS is highly likely to result in recapturing the huge amount of legal taxes they miss, if they were only able to. Clearly the status quo is insufficient to actually collect the taxes currently on the books. Of course tax law in the US is also politically designed to make policy that results in favored industries and organizations and to make this as obfuscated as possible, resulting in crazy complexity, which of course also benefits the tax prep industry.