How easy is it? The system here in Australia seems super well done. They already have all my data. I just look through it and confirm its correct and add in any deductions I want to make.
The IRS probably could provide a more or less pre-filled form for many people. They have the information on most taxable income already. However, they dont. The reasons for this are complex, like the tax code, but mostly boil down to "politics".
[0] Here's the main tax form: https://www.irs.gov/pub/irs-pdf/f1040.pdf
For federal taxes, payroll systems typically transmit your data to our taxation office on each payrun, and then a few weeks (or sometimes, immediately) after the end of the financial year, your details are already in the system and you just need to hit the button to submit.
Our tax office has also tried to make it easier for individuals with simple requirements to keep records of work-related tax deductions, etc, such that you can use a mobile app to record them, and then it can also be imported into your return with a single click.
I think for many people - myself included - it's easy enough that you can (and I have) complete it on your phone on the way to work on July 1st (the first day of our income tax year, and the first day you can file for the prior year).