Part of the problem is unbalanced incentives between tax inspectors and crooked accountants.

If an accountant​ helps a billionaire dodge taxes he can earn millions of dollars in fees. If a tax inspector exposes the scam he might get a salary bonus worth a few thousand dollars.

I would like to see tax inspectors get paid by commission. If the tax inspector exposes a scam and the government collects $50 million, the inspector gets $2 million. (And to avoid overzealous inspectors, if a tax audit finds no impropriety the inspector forfeits $20,000)