It's not about the cash per se, it's big brother wanting to know how you got the money in the first place. If you can show it's all legal and taxes paid then no problem cash is fine. But of course as soon as you mention cash, eyebrows are raised "why would someone want to keep that much cash and not want to put it in a bank if they are all clean?"
Big brother wanting its lion share in everything you earn is the crux of it.
When your an immigrant and your money is from places with bad records, they can't really push too much beyond it, or tax you for money made out of the country where you were not a citizen or resident of the USA. Shitty undeveloped countries are essentially a laundry for cash in general.
Big brother wanting its lion share in everything you earn is the crux of it.