But all of that complexity seems to be added with good intentions. Just look at the "Coronavirus Tax Relief" as the latest example which by itself has a long FAQ page describing all of the details: https://www.irs.gov/newsroom/coronavirus-related-relief-for-.... Adding that to the tax code was probably the best short-term approach to quickly ship something (and lots of people are probably in favor of those changes), but that complexity adds up over time.

To me this looks somewhat like the broken windows fallacy: In countries where the tax code is simple and allow for "automated" taxes nobody is going to advocate for a new law if it makes filing taxes a lot more complicated. But in countries where the tax code is already complex adding one more complex law doesn't make too much of a difference.

Some of the complexity is also due to the design of our governments. I don’t know much about taxes in Spain, but I’d guess the number of income tax jurisdictions there is not a 3 digit number.