Many trading platforms in Denmark do automated reporting, and some gig-economy apps do automated reporting too. But if you setup a lemonade stand, well, reporting is in you :)
The tax authorities also have pretty good phone support. It's a bit overloaded around filing season (but getting through is possible), but if come home from an internship abroad and call them, they are happy to tell you where and how to report income and taxes paid in Canada for example.
I bet it's expensive to offer phone support. But if you want people to pay taxes the best way to do it is to make it easy to do the right thing.
This advice goes pretty much everywhere, if you make it easy to do something, people will do it :)
At any time, you can log in and easily see how much the government thinks you have earned and from who, and which tax rate (tax code) is being applied to you. You can correct this as well if it is wrong.
Having lived in Australia, the US and UK, I've found the UK system to be the easiest to understand and navigate. Australia isn't too far behind. The US is just a mess.
However it is not a "one click to accept the default" system because there's still a lot of important information it does not know.
But nevertheless the yearly filing process is rather straightforward, as the tax authorities provide their own free web app for this purpose. It hasn't always been great but it improved significantly over the last few years.
There are no deductions, for a lot of stuff you just get money directly from the government, as an incentive.
Not sure how IRS is going to know how many dependents I have living in my house or whether I've lost my hearing and am now deaf.
The IRS could use easily use last years data plus what was submitted by companies to pre fill most Americans taxes accurately and people could amend those forms as needed with less effort than starting from scratch.
Yes, EZ file should work for 99.9% of the US. But it doesn't, because the tax code is horrendously complex
IRS could possibly reach into some of that. Or it simply relies on what you reported last year.
But sure, you'll have to correct some details every few years. This might also be the case if you have income from bitcoin, etc.