[runako]

Really educational comparison!

It seems the major difference derives from our (American) punitive approach to those who use our meager social safety net.

For example a large swath of Americans earn an income that entitles them to assistance in the form of the Earned Income Tax Credit (EITC). This is (roughly, it depends) available to people who earn < 85% of the median income. But they have to file taxes to get the money they are owed (because we hate the poor in America and this will dissuade them from getting their money). So that's going to be a large set of the country that has to apply or leave money on the table.

For a large set in the middle class, you have to file because you leave money on the table by not claiming deductions.

So even if we weren't all more or less required by law to file, we would mostly have a financial incentives to file anyway.

Oh and anecdotally, side-hustles and second jobs are very common in the US. Poor social safety net, no employment contracts, very low minimum wage, high healthcare costs all doom most Americans to perpetually precarious financial circumstances. So everybody is trying to get a little more so they don't get wiped out.