All employers report income and deduct taxes from paychecks directly, so the government knows how much you have paid.

What they don’t know is if you’ve won a lottery, sold a service or have someone bunking at your place paying rent. This you need to declare yourself.

In the UK the rule is: we will assume you have nothing else, please let us know if you have additional income

In the US (and Canada) the rule is: we will assume you have a lot of shit, please let us know even if you don’t have additional income.

The US system stems from Conservatives having nothing other to run on than “people are going to grift and if you’re an honest employed american you have nothing to worry about”. Which is a measure that mostly affects the poor and those who don’t know the law.

The same reason healthcare was tied to employers. They convince people nothing could go wrong because only those working should have healthcare. Which we can all see is insane but looks good on paper.