Besides, as the old Madam said, "How're gonna figure the amount to tax? Ya gonna put a meter on it?"
But that's how they finally got Al Capone. They couldn't get him for murders or bank robberies or prohibition violations, so they figured they'd look into his string of brothels.
They ended up (perhaps among other things), intercepting the dirty laundry that was sent out. They figured one piece of business resulted in one change of sheet and I think it was two towels. Converting the laundry count back into business volume, at the appropriate rate per John, they came up with the approximate gross revenue. They worked from there to taxes due, and Scarface Al went to jail on tax evasion.
As the old adage has it, "The power to tax is the power to destroy."