The "hard / impossible" part is that the taxing unrealized gains requires a policy around recognition of unrealized losses, and a policy in either direction has empirical and well-understood adverse consequences. Governments generally avoid tax policies that unavoidably have nasty and unmanageable adverse consequences that exceed the value of the revenue generated.
If you do not recognize unrealized losses, it strongly biases investment toward low-risk rent-seeking investments because the expected return is much higher than long-term high-risk investments under this tax regime. Startups, biotech, etc become unattractive as a place to put money because the financials don't make sense relative to rent-seeking and investment is not a charity. Paying taxes on non-liquid assets is a massive risk for average people, greatly reducing the practical investment opportunities available to them.
If you do properly recognize unrealized losses, it creates a scalable new mechanism for tax avoidance that would reduce tax revenues. Not only are valuations strictly notional and effectively fictional for many assets, but intangibles are often entangled in asset values that can't be accounted for in a sane way (e.g. the act of transferring an asset can greatly reduce its value ipso facto). These intrinsic logical inconsistencies allow almost any tax story to be constructed using assets.
While no government wants to deal with the consequences of broadly recognizing unrealized losses, they also don't want to incentivize all investment to go into low-risk rent-seeking endeavors like real estate instead of long-term high-risk investments in technology and innovation. It is a damned if you do, damned if you don't, so governments generally opt out of this type of tax altogether.
But you're still discussing unrealized gains/losses, when I'm suggesting to ignore them. If someone is borrowing millions, hundreds of millions, or even billions against untaxed collateral, then they should be taxed on what they borrow, limited to how much they can borrow, and/or taxed more on what they buy. It's their and the bank's problem if they're borrowing against unrealized gains or losses.
The wealth inequality problem in the U.S. is INSANE. If the U.S. had a population of just 10,000,000, literally 12 people would have as much wealth as the bottom 60%, i.e., 6,000,000 people, combined. And yet, here we are talking about how "impossible" it is to simply tax wealthy people.
These systems were made by humans. They can be fixed. Wealth inequality and taxation of the wealthy are not laws of nature that can't be changed.
If you do not recognize unrealized losses, it strongly biases investment toward low-risk rent-seeking investments because the expected return is much higher than long-term high-risk investments under this tax regime. Startups, biotech, etc become unattractive as a place to put money because the financials don't make sense relative to rent-seeking and investment is not a charity. Paying taxes on non-liquid assets is a massive risk for average people, greatly reducing the practical investment opportunities available to them.
If you do properly recognize unrealized losses, it creates a scalable new mechanism for tax avoidance that would reduce tax revenues. Not only are valuations strictly notional and effectively fictional for many assets, but intangibles are often entangled in asset values that can't be accounted for in a sane way (e.g. the act of transferring an asset can greatly reduce its value ipso facto). These intrinsic logical inconsistencies allow almost any tax story to be constructed using assets.
While no government wants to deal with the consequences of broadly recognizing unrealized losses, they also don't want to incentivize all investment to go into low-risk rent-seeking endeavors like real estate instead of long-term high-risk investments in technology and innovation. It is a damned if you do, damned if you don't, so governments generally opt out of this type of tax altogether.