Is the IRS well known as a barely functioning behemoth? There's no shortage of animus towards them, but it's generally seemed closer to "I don't like the agency that takes my money" than "this agency is incompetent."
They're seriously understaffed. Forget about the partisan politics for the reasons behind it but a terrible side effect of this lack of resources is that they only go after the low hanging fruits to audit. Like Joey Banana who forgot to report a few hundred dollars of income will get audited but complicated tax evasion schemes go unaudited.
Isn't a lot of the reason that they are impractical that there are powerful organizations lobbying for taxes to be painful and complicated to pay. E.g. Turbotax and the Americans for Tax Reform
