But your point is wrong. In 87% of the cases, the IRS does have all the information. Even if you have donations or other deductions it doesn't matter, because 87% of people take the standard deduction since it's more than their itemized deductions.

And marriage and death records are public, as are probate. So they would have all of that too.

There's really no point in continuing this discussion if you think the IRS is scraping data of every county in the US for marital status changes.

Of course they aren't. But they could if there was automatic billing.

Or simply ask you if the most common situations apply to you, just like turbotax does.

That's not what they said at all. Most people are not changing marital status or starting businesses every year.

The IRS could send a tax bill for what they do know, with an option to agree that is all you owe and pay, or an option that you will need to file the taxes yourself because they are missing information.

For the vast majority of Americans, option 1 will cover them.

I think it’s likely the IRS uses some kind of database of US vital statistics (including marriages) as part of efforts to detect tax fraud. Other federal agencies do, eg the State Department’s passport office uses a proprietary database called EVVE of birth and death data. [0]

[0]. https://www.naphsis.org/evve