> an agency which _already has that piece of paper_

Sooo... the IRS knows that you installed solar panels last year and will be taking an energy credit for them? They know how much bitcoin you sold? How much mortgage interest you'll be deducting? The strike price of the stock options you exercised in your private company, and their current fair market value?

There's a bajillion variables that go into filing a tax return, and only you are the one that knows all of them, and sometimes not even then! This idea that the government "already knows" how much you owe in taxes is a complete fiction.