A Norwegian "ENK" ("enkeltmannsforetak"; self-employment) deals with a more integrated state reporting environment, stricter cash-sale controls, more emphasis on formal bookkeeping and VAT/cash-register infrastructure, and a more pre-filled tax ecosystem.

You can get a long way cheating the system if you deal with cash only, as banks etc. are required to report everything about everyone to the government, but these days it can only take you so far.

My understand is that the US is much more depending on self-reporting.

But given that the US has its own industry involving tax reporting, and having lived there myself, I don't believe you when you say it's "simple." ;)