If it was automatic, income would be missed. The government would be revealing what it doesn't know about, and people would not supply the missing information.
Under the current system, people aren't sure what the government is aware of. Because we get in trouble for leaving things off, we are more likely to report everything.
Make people file a tax return, but auto-populate it with the data the government already knows. And at the end include a tick box clause that essentially says "I have included everything and reviewed the autofill info, and if I have omitted anything later discovered I will be prosecuted".
20 seconds to file your taxes if there's nothing unknown. Very easy to add extra info. Covers the tax departments back of you intentionally mislead them.
Same here in Europe, my tax form is auto-filled I only have to check the numbers and confirm, with same warning and laws to back it up in case you decide to hide income.
We know what the IRS knows because the majority of people get the same forms as the IRS as far as income goes: W2, 1099, 1098, ... Unless you’re self-employed, the IRS only misses some deductions or foreign income.
Ever performed a lot of stock trades in a single year and had the IRS believe that you owe them $5 million in income taxes because they assume every sale you made was all profit? That's always fun, especially when they think they're right and they're entirely wrong.
The IRS information is at times comically incorrect in fact.
If you're trading more than $20 million a year in stocks, you are in the vast minority that everyone here acknowledges would need to adjust and file your taxes manually.
99.999% of the US population is not trading $20 million a year in stocks, and everyone else here is merely talking about the 95% of the population that has straightforward tax returns.
Which is why most countries have a system that shows you what they think, and you can accept or not. It’s not like the majority of people are in such a situation, and likely over time that will become less common as its often a byproduct of legacy systems / mergers that lose the original transaction.
Under the current system, people aren't sure what the government is aware of. Because we get in trouble for leaving things off, we are more likely to report everything.