[coderintherye]

You assume everyone itemizes deductions. There is a thing called the "standard deduction" which renders your point moot for the people using it which I suspect has a lot of overlap with the people who would like the IRS to do the calculation.

[NovemberWhiskey]

No; I am not assuming that. The standard deduction does cover a lot of people; but a lot of the people that it covers end up filling out other forms as part of their returns for tax credits.

For example, the Earned Income Tax Credit is a means-tested benefit program that's delivered through the tax system. This is not a niche program - 25 million families benefit from it.

It requires filers to identify how many qualifying children they have, which in turns depends on things like the educational status (is this child in school, college, are they a full-time student) and residency (child must generally live with you, but with exceptions for overseas military service, etc).

In cases where multiple filers can claim a qualifying child (e.g. cohabiting single filers), you're required to make an election as to which return will claim the qualifying child.

The EITC also encourages reporting of informal income (because it's only available to people who earned an income) which might not be on a W-2 / 1099, e.g. baby sitting, to qualify.

None of that stuff is known to the IRS from W-2s, 1099s etc. and they are dependent on the filer to provide it every year, especially as circumstances change; and filers are strongly incentivized to provide it as the credit is refundable.