I think you're misinterpreting the GP's point. Clearly, at least in our current system, it is essential to tell the IRS the parts of the return that they don't already know such as what are your expenses, deductions, marital status, etc.
But the absurd thing is that the capture of the IRS by the paid tax prep scammers has prevented them from simply showing you what's on your tax transcripts and having you click "Agree" or "Modify" for each one. Instead, you get your own copy of the 1099-B, 1099-DIV, 1099-INTs, and are administered a pointless "honesty test" to see if you'll type in the same numbers they have, or be automatically punished.
Obviously, Direct File was ideally situated to offer this feature since IRS has the data themselves, and simply populating the numbers is a highly efficient way of ingesting the data into your return.
Under Biden, the IRS tried to make w2s and 1099s available. If you log into the IRS website with your information, you can download the w2s and 1099s the Service has in your name.
Antitax activists have fought these steps every step of the way because the less annoying tax filing is, the less people will buy their antitax arguments.
The IRS isn’t captured by these predatory tax preparers, Congress is. The IRS can’t do a lot on data without Congress specifically authorizing it. And the Republican Party is in bed with the antitax activists who are in bed with the tax preparation companies.
Use Form 4506-T to request them.
(They can detect that you had a sale for which they got a 1099-B but you didn’t list on your Schedule D. That doesn’t mean they have enough information to fix it.)
You mistakenly assume that simply knowing what is on the 1099-B form is sufficient to determine your tax on the gain. They don't know if you are married or single or head of household (filing status) in the current tax year. They don't know what some of your itemized deductions and other income not reported to them might be (which in turn, along with filing status, determines what marginal tax bracket you are in). They don't know if you are actually just a nominee for someone else's income. These are just a few examples. They don't know any of this stuff until you tell them by filing your complete return.