Earlier quoted context omitted.
Why is that a stupid rule? If I sell Nvidia stock to buy AMD stock, I need to pay tax on my Nvidia stock gains.
Because, unlike at the stock market, the government provides no assurances regarding any part of the crypto trade that is being made. When things go south, fraud happens in crypto, government is pretty much zero help in most countries of the world at most times. I imagine if it turned out you never possessed any AMD or Nvidia stocks and all of that was just fake number on a fake piece of paper there would be serious…
You example about trading equities (gifting? sharing?) is wrong. You owe tax on trading your stock for another stock. Even if you circumvent a broker and deal in literal certificates. This is a disposition in the eyes of every major tax authority. There are very few exceptions (rollovers or corporate reorganization are some examples). Difficulty in calculating the FMV of the disposition is entirely your problem.