Do they know how much you'll spend on sales tax for a given tax year? How much you'll pay out of pocket for health expenses? Do you?

These are not "interesting", but nearly everyone has 'em, and the IRS cannot know, without bending space time, how much you'll owe because of those deductions (assuming an itemized deduction, though they're still in play either way because they can help tip the balance between taking a standard or itemized deductions).

> assuming an itemized deduction

Only 13% of all US federal tax filings each year itemize deductions. That means it would help the vast majority of filers and not harm the rest in any way whatsoever. Weird thing to be against if someone isn't misanthropic.

I was pointing out to the GP that the IRS cannot know the final amount due because of the potential for some very common types of deductions.

I wasn't disagreeing with the idea of pre-filling tax return data.

But just imagine if your starting point for your taxes was "here's 95% of your information already filled out, got anything else to add?" Wouldn't that be even easier? Nobody is saying you can't itemize if you want, but for the vast majority of taxpayers the standard deduction is a larger deduction than itemizing and far lower risk of audit.

It'd be fantastic, and I agree. I never said otherwise.

I was simply offering examples of "normal" deductions that can easily change what you owe, and how those deductions aren't as uncommon as the person I was commenting to would have you believe.