I've actually had this discussion quite a few times (for someone in a field not at all related to taxes)

Where it gets "complicated" (aka no longer a "one click sign and verify") is when you start adding things that are "nonstandard" - "standard" meaning single w2, standardized deduction, etc. According to Intuit[0] as well as other research[1], only ~40% of US taxpayers follow this format. This is compared to the 87.7% of surveyed tax administrations[2] (which does include the US) that have the infrastructure to pre-populate basic wage and salary data. The difference is that in countries like Denmark or Sweden, that simple data represents near-100% of what is needed for the vast majority of citizens.

So for example, even if the US might know exactly how much you made on a 1099 (which is freelance, independent contractor, etc), they wouldn't know your expenses. Oftentimes though, even if you do have these more complicated returns your W2 (which is the "simple" part) can at least be auto-filled. It's just the rest of the stuff the government doesn't necessarily know about.

(note: I'm by no means a tax expert and may be misinterpreting this data)

[0] https://turbotax.intuit.com/personal-taxes/online/free-editi...

[1] https://www.brookings.edu/articles/the-simple-return-reducin...

[2] https://www.oecd.org/en/publications/tax-administration-2025...