And the IRS has most of those numbers (if not all for most people). The process should start with the IRS saying, "Here's what we have so far...". Not "We know but we won't tell you. Don't fuck it up cause them we'll screw up."

The process is intentionally designed to insight fear.

In countries like the UK, the tax code is simple enough that most employees never have to file a tax return. There isn't a long list of deductions and exemptions, so unless your financial affairs are particularly complicated or unusual, everything is handled by your employer.

And adjust, in one way or another, the amount of those deductions. Even if you don't itemize, the amount taken out of salaries for most people between Federal and State doesn't align with what was taken out of their salaries. (For a lot of people, they're actually owed a refund though.)