It is truly because the IRS wants the tax income. The IRS actually cannot legally disclose any taxpayers return information to a law enforcement agency unless the law enforcement agency gets a court order granting them access. The important thing there is that the IRS can't tip off law enforcement, law enforcement has to be the one to initiate the request. The only exception to this is if terrorism is involved. It still requires a court order but the IRS can be the one to request the court order.

This only applies to your actual tax filings though. If you get audited, any information they gather through interviews etc can be turned over to law enforcement if there is evidence of a crime being committed. That is probably a functional workaround. If you report illegal income on your taxes, you'll get audited and the witness testimony from the audit is turned over.

https://money.cnn.com/2013/02/28/news/economy/illegal-income-tax/index.html

Mercedes AMG reached 50% thermal efficiency with their Formula 1 engine about 5 years ago but your point about no “real” engine is still very valid. The F1 engine is using hybrid tech to reach that and is more expensive that many homes, let alone cars. They also are not using standard fuels or oils either. The engine reaches that thermal efficiency not while idling or going at low speeds, it does it at full throttle producing over 1000hp so, while fuel efficient in relation to other F1 engines, I wouldn’t be surprised if the semi truck used to transport the cars from track to track get better MPG.