https://www.propublica.org/article/filing-taxes-could-be-fre...
California piloted a pre-filled tax return system (similar to Sweden and Australias, that the OP was talking about) called ReadyReturn in 2005 with 51,000 people.
Intuit then spent $3 million to smash it. They lobbied and then sued the state to stop the program. They are also profoundly incompetent technologists. There's an entire subreddit dedicated to how broken their stuff is (https://www.reddit.com/r/TurboTax/top/?t=all)
What did the participants of ReadyReturn, the pre-fille forms, think of the government program that Intuit killed?
"99% stated they were satisfied with ReadyReturn, 97% stated this is the type of service government should provide, 96% stated it was more convenient than how they filed in the past, 95% stated it saved them time, and 98% stated they would use it again"
Once again private interests sabotage and destroy the public will.
The line of "the reason taxes are so difficult..." is from sheer propaganda shills. Tax simplification has happened, it worked, and Intuit killed it every time. The reason is Intuit. They are the buggy whip industry that won. Shameless grifters that intentionally disrupt people's lives for profit.
Use another service. Any listed on the IRS website: https://apps.irs.gov/app/freeFile/browse-all-offers ... Never give Intuit a dime and click on every ad you see from them.
That said, there are a lot of challenges to such a program.
Ready return was limited to people who were single, no-dependents, standard-deduction, only-wage-income, one-employer, with a maximum adjusted gross income of $139,917
This is great for a number of people and should exist!
That said, if you fall into this category, your 1040 should take 15 minutes to file by paper.
Where automated systems break down is handling more complex scenarios. Selection of deductions, pass through business income and losses, charitable donations, corporate investment vehicles, ect.
Many of this things require individuals to make choices at the time of filing. This is challenging to automate.
There are some exceptions to this depending on business size i think but the vast majority of the data is in the feds hands.
It is incredibly arcane, and in some cases individuals holding publicly traded assets are legally required to file income tax in all 50 states.