In my country, it's automatically done for those with "regular" incomes, even end of year tax back is as simple as logging on to Revenue and clicking a button.
You're only on your own if you're self employed, generate non-standard income etc (like shares, which I imagine most people on HN are).
In those cases, the government does _not_ know how much you earn until your audited. And audits are neither automated nor simple, a dedicated auditor is tasked with identifying all your sources of income and verifying everything. Point being it's manual.