I'm not sure about that 90% figure, seems a little high.

25 million people receive the earned income tax credit (i.e. they are poor), and are at high risk of audit https://www.propublica.org/article/irs-now-audits-poor-ameri...

9 million are expats, for whom US tax filing is a special type of hell

15 million are self-employed, which means lots of extra filing https://www.bls.gov/spotlight/2016/self-employment-in-the-un...

putting us at circa 50M out of circa 167M returns filed https://www.irs.gov/newsroom/filing-season-statistics-for-we...

means US tax returns are rather complicated for 30% of people before we start to consider high-income earners.

Only something like 15-20% of Brits actually have to do a tax return in the first place. I did mine for the first time this year, and it was surprisingly easy.