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 government involvement pretty much everywhere.

Also on more practical note, a single sale of crypto can really be a 50 sales to 50 people at various prices. Calculating gain is virutally unworkable. Especially between crypto pairs where the value of one or the other isn't really known.

When you sell NVidia stock and buy AMD it's a sale and a purchase. But if somebody gave you some AMD stock for your NVIDIA stock, there's no sale or purchase and the value of each is only a guess.

That's why reasonable countries decided it's way better to tax stuff on exit to fiat, via conversion or purchase. And it works just fine. When it's crypto it remains Miki Mouse money, but when it exits to fiat and there's more of it than was put in, there's real and taxable gain.