A single sale of crypto can really be 50 sales to 50 people at various prices. Calculating gain is virtually unworkable. Especially between crypto pairs where the value of one or the other isn't really known and changes every second and depends on particular exchange out of may you could be trading on.

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 of something other than crypto. 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.

Taxing crypto on every trade would be like taxing forex on every trade.