This is for the people who right now are paying some company to essentially and unnecessarily siphon off some of their tax money.
Obviously. You should read categorical statements like this one as, "The IRS [often and in many of the most common cases] have all the data."
That's a generally-applicable, useful, mental substitution that saves us from unnecessary and constant waffling and equivocation in everyday conversation.
So they only know your revenues, not your expenses. And this applies for business as well as personal.
Lots and lots and lots of people don't have any expenses. They don't itemize. They don't want, have, or need any of the stuff you're protesting about. They have a job. And that job reports their income to the IRS. End of story. Case closed. That's it. That's their entire situation.
You're imagining another kind of person, but I'm not talking about that other kind of person.
And, in any case, the question isn't, "Will the IRS ever make a mistake?" Obviously they will. The question is, "Will the IRS make fewer mistakes than the mass of folks who don't itemize and who have simple tax situations?"
I think that's clearly true.
Similar, yet ... stupider...
Did some standard stock buy/sells in a year, and the brokerage reported all the sales as full income, and no original cost, so they'd reported that I'd had something like $120k in 'income' ... No penalties for them for filing bad data, but it took me a while and some CPA time to respond accordingly. :/
Not..entirely true.
My 2016 taxes were professionally done, but we somehow forgot to include one of the something-something-1099 forms from a brokerage firm. The brokerage reported some RSU stock-sells to the IRS but didn't report the cost-basis, and so the IRS assumed the cost-basis was $0 and thus taxed the full sale as income (effectively double-taxing all the RSUs). They just sent a tax bill for something like $20k plus a bunch of fees and interest.
It's an accounting-error so we won't owe much, but I doubt we would have seen this mistake if the IRS had just told us a total amount to pay. We have two tech-job incomes both of which are highly-dependent on equity, so we'd almost certainly end up getting a CPA to run all the numbers again anyway.
I'm a fan of reducing bureaucracy of double-reporting tax info, but I'm personally unwilling to trust that the IRS and various banks / brokerage systems are up to the task of generating trustworthy data (and/or erring on the side of the taxpayer in situations like this).