Its not terribly difficult if you only have say wage income and some interest on a bank account [0]. But, it can get very complex very quickly if you have other sources of income, or want to be sure to take all of the deductions you might be eligible for. On top of that, each state has their own set of taxes (though in my experience, they are simpler and largely rely on information from the federal taxes). Hence why most people use software or pay tax professionals to prepare their taxes for them.
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".
I think that it's straightforward enough in Australia, but that's partially because we file and pay only federal taxes, with states levying payroll tax, land tax (or stamp duty), etc, but those are less frequent transactions and generally accountants are involved.
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).
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