My understanding is part of the problem is that they have not been well-funded enough to even think about going after the top earners, who can easily afford to outnumber them with lawyers and CPAs. So, they give the people at the very top a pass because even if they are tax-dodgers, it's too expensive to prove it. Which leaves them going after middle class people who exaggerated their charitable deduction a little or hid a $100 eBay sale. They're easy marks and won't be able to resist with an army lawyers.
The reason they don't go after the top is because the top use expert tax lawyers to file. Anything they do that is shady is ambiguous in the law and thus had a theoretical basis. The IRS isn't taking on those people because if they lose in court, they lose the people who were paying extra by not exploiting the loophole in good faith.
Also, they say they are focusing on entities making over $400k/yr. I'm sure there are exceptions and trust issues on that statement. We'll see.
