Earlier quoted context omitted.
Data is always crap, but there are error bars. GDP is pretty well accounted for. There was shadow wealth at every point of the last decade, so it's already factored in, the numbers (and studies) are already taking that into account, ... unless! Unless folks suddenly started to commit more tax fraud. (Which is hard to do if your money comes from capital.) https://budgetmodel.wharton.upenn.edu/issues/2021/4/30/map-i...
Yea well there is you answer. Most wealth is not money.
The US is still the wild west in many regards (from good old identity fraud to sheriffs/judges/prosecutors with license to bully anyone without the right connections) and of course the basically institutionalized various other scams come to mind (thriving MLMs, tax exempt religious orgs and megachurches, doctors turned sales-reps for opiates, payday loans, untouchable plants and mining operations polluting and leaving deadly open pits, you know ...), but if there's one thing you can't really fault the US for is getting lazy on private property regulations. And neither the IRS nor the CA FTB is known for being completely clueless.