Somewhere around 90% of Americans take the standard deduction and the IRS is in a better position to judge eligibility for most of the adjustments outside that

Somewhere around 90% of Americans take the standard deduction and the IRS is in a better position to judge eligibility for most of the adjustments outside that (e.g. EITC) than many taxpayers are. There's always going to be a way to fill out your own taxes for people with complicated tax situations, so there's no harm in simplifying things and reducing mistakes/fraud for the vast, vast majority of people.