Similarly, I doubt the IRS wants taxpayers to know how much it knows. What if the IRS has no idea about someone's Craigslist hobby fixing and reselling old comp

Similarly, I doubt the IRS wants taxpayers to know how much it knows. What if the IRS has no idea about someone's Craigslist hobby fixing and reselling old computers that technically should be considered taxable and add 10% to their income? They probably prefer the ambiguity about how much they know.