So, every year, you have to do a bunch of paperwork (it took me over 4 hours this year) to calculate your actual tax obligation, and then either you send the government a check or they send you a check to settle the difference.
For example, the existence of tax-advantaged savings vehicles such as IRAs and HSAs mean that the taxable portion of your income isn't settled until the deadline for making contributions for that tax year. Which is April 15. But I imagine there would be a lot of popular political backlash if Congress were to abolish IRAs in the name of sticking it to Intuit.
There's also that whole mess of expenditures that you can deduct from your taxable income, which doesn't kick in until the total of your deductible expenses exceeds the standard deduction. And a lot of people were worried when the standard deduction got increased a few years ago. It made a lot of people's taxes nominally easier to calculate, but people were worried that it might remove an incentive for charitable donations, or reduce the largess that the government gives to homeowners relative to renters, or alter the impact of tax incentives for people who put solar panels on their roofs, or whatever.
Long story short, we love to blame Intuit, and it is true that Intuit generally wants a nasty complicated tax code, but it's also true that, in the aggregate, so does America.
The (previous) existence of the 1040EZ showed that a lot of people have simple returns that the IRS could do for them.
That's partly because banks send all of their information to HMRC.
For example, one could tweak the deadline for contributions to those accounts to be say March 1, make the financial institutions report to the IRS by March 15th, send everyone their estimate on April 1st and have a deadline of April 15th to confirm or adjust their tax filing.
Most of the big things that involve deductions are already tied to a financial institution that already report to the IRS.
But the tax system is very complicated, with multiple tax brackets (income above a threshold is taxed at a higher rate), deductions, exemptions, exceptions, alternative-minimum, and more. There are deductions and exceptions for things like mortgage payments that go to interest (on only one residential property), donations to certified charities up to a limit, green energy incentives, travel to start a new job, who knows what.
So, we have to settle-up with lots of complicated forms each year, and I've tried alternatives to Turbo Tax, but didn't have enough confidence with the alternative's handling of all the complexity, they just were noticeably worse at handling all the complex details. Not that I like Turbo Tax or the overall situation, but for me it's either Turbo Tax or pay a more expensive accountant.
So at the end of each year you need to go through all the income sources of that year, subtract all deductions and compute how much actual tax you own. And pay or receive the difference to/from IRS.
Even if you have an extremely simple income situation (only one wage, no other income), depending on the state you live in you may still qualify for deductions that the employer is not aware of so it can be in your advantage to do the taxes.
[1] https://www.investopedia.com/ask/answers/07/taxtipfederal.as...
[2] https://www.irs.gov/help/ita/do-i-need-to-file-a-tax-return
https://www.propublica.org/article/inside-turbotax-20-year-f...
Most Americans are simply unaware that other countries have dramatically simplified this problem. It's amazing how many have magically assumed this was because this was some socialist concept and the immediately launch into "oh, but how much more in taxes are you paying? pfff."