The most important is taxation. People pay their babysitters or gardeners under the table, or transact with friends and family without reporting income, and this is a huge amount of lost tax revenue.
Another reason are policy options. For one, there are certain decidedly "non-terrorist" goods and services that the government might not want you to purchase. Heck, in the era of ZIRP, many economists were seriously talking about negative interest rates. You can't do that if a person has the option of taking out cash and hiding it under the mattress.
At some point I noticed that her shows were starting to significantly diverge from her area of expertise and she was weighing in on much broader topics, something in her early shows she often criticised scientists for ("don't think because someone is an expert in A that he can judge B").
At some point she weighted in on some topics where I'm an expert or at least have significant insights and I realised that she is largely talking without any understanding, often being wrong (although difficult to ascertain for nonexperts). At the same time she started to become more and more ambiguous in her messaging about academia, scientific communities etc., clearly peddling to the "sceptics" (in quotes because they tend to only ever be sceptic towards towards what the call the "establishment"). Initially she would still qualify or weaken her "questions" but later the peddling became more and more obvious.
From what the article writes I'm not the only one who has seen this and it seems to go beyond just peddling.
I think it's a lesson that we all consistently fail to apply to ourselves. It is so pervasive on social media - HN included - yet it's something we only attribute to others. Our hot takes on quantum physics, molecular biology, and economics are always reasonable and rooted in keen insights.
It happens for a reason. There's something deeply satisfying about being a contrarian: the implication that you're smarter than the masses. It's usually hard to be a contrarian in your primary field of expertise. It's a lot easier to be a contrarian in someone else's.