That's just it though - the income tax code is byzantine.
A restaurant charge is simple - here's what you ordered, here's what each thing costs, here's the total. If you really suspect shenanigans, you get a copy of the menu to validate against.
Property tax is usually pretty simple, as is vehicle registration. Both can be described in a couple of paragraphs.
The problem right now is that even if the forms were precalculated, the IRS doesn't necessarily know about the deductions you're entitled to, and claiming those is a far more intricate process than simply filling out your PII (1040) or your wage info (W2).
Here's a relevant example. I just started going to school again at the end of last year (November). A run through with TaxAct/HR Block showed me as owing about $600 before any deductions or credits - you have to tell them about those. The fact that I started going to school gave me the "lifetime education credit", and turned a $600 payment (that I could not afford at the time, and was somewhat stressed about) into a roughly $2K payday.
Without the software, or paying an accountant, I wouldn't have known about that, and paid $600 that I didn't need to pay.