The part that they can't do correctly is mostly deduction-side.

I bet that they can get it correct for 90% of the taxpayers. They certainly could do it for me, using sensible defaults.

(e.g. mortgage interest is reported, real estate taxes are reported, local sales tax is a calculation in most cases).

Where they're not going to get it right is if you've got a Schedule C, since none of that is reported elsewhere.

For simple purchases of common stock, the purchase price is NOT reported to the IRS. Although, many brokers may provide it for your information.

I believe (meaning, I've got a form with that data) that that has changed as of this year, at least, they are now reporting the cost basis for stocks that have been sold. I don't remember seeing this in previous years.

Yep, cost basis is now being tracked and reported.