"Estimate" implies someone is trying to... estimate something. The IRS has partial information. They don't have anything approaching something that anyone could reasonably call an estimate, and this isn't just an example of HN pedantry, it's an important distinction.
They know some of your income, but not all of it. Your 1099 income for the year is getting filed by your clients at the same time you're filing it, so all they know is what your AGI was last year. They know your expenses from last year in terms of the numbers, but they haven't been checked (last I saw, they were 3-5 years behind on cross-checking and validation depending on type of return). So they could plug the number in, sort of.
So they have your reported info from previous years, and a best guess at some of what this year's information might be.
So give me a partially filled out form with the information they do know then. Fill in the standard deduction, let me do the extra credit work for itemized deductions if I really want, eg I find taxes fun to do. (Some 90% of filers just go with the standard deductions.) Don't buy into Intuit's nonsense that the current system is good for anything other than their bottom line.
So the IRS calculates for each filing status, and you choose one. This would be a lot to ask years ago, but nowadays many citizen-facing government processes are web apps. Think TurboTax pre-filled with whatever has been reported.
A best effort calculation done with partial information is a type of estimate.
I agree, this won't work for a big class of people. It won't work for me. But it will work for the majority of tax filers who don't have a 1099 and who don't file a schedule A, B, C, D ...
My tax return last year was a half inch thick, printed out. I had to file on paper. I very much understand that this doesn't work for everyone. But it works for most people.
They know some of your income, but not all of it. Your 1099 income for the year is getting filed by your clients at the same time you're filing it, so all they know is what your AGI was last year. They know your expenses from last year in terms of the numbers, but they haven't been checked (last I saw, they were 3-5 years behind on cross-checking and validation depending on type of return). So they could plug the number in, sort of.
So they have your reported info from previous years, and a best guess at some of what this year's information might be.