This is similar to income-shaped small transfers of money within families; there would be Congressional hearings if the IRS found a deficiency in mandatory withholding taxes on babysitting money, which is unambiguously earned income of a statutory employee.
Maybe because the IRS declared years ago that cryptocurrencies will be treated similar to other commodities?
And while I don't know the tax code, I suspect if you convert your USD into rupees, and those rupees suddenly increased value tenfold, and you started buying Mercedes with them, the IRS likely will come after you there as well :-)
Crypto is basically different because the high volatility resulted in large gains being realized.
Source: publication 525 https://www.irs.gov/pub/irs-pdf/p525.pdf
So why shouldn't the IRS do the same?
Many people do not treat bitcoin like gold, they treat it like a paypal account.
It is much more impractical to track the value of fluctuating (maybe x1000 more fluctuations than the USD?) cryptocurrencies though.
I travel to India every year, and I always carry with me a few hundred USD worth of rupees (the local currency).
It would be insane for me to try and track the value of the rupees (in USD) for every time I bought something, and calculate the deltas between that and what I originally gave to the money changer. It is completely impractical to do that, and while I'm not an accountant, I have an intuition that for any reasonable amount of money that any traveller would be carrying with them, the fluctuations are irrelevant.
And yet this is what people using cryptocurrencies are expected to do.
Obviously if there is a large capital loss or gain, then this should be reported on your taxes, and I suspect the same is true for foreign currency exchange.
I guess to ask a question: are individuals expected to track the fluctuations in USD value of their cash while travelling? If so: is there a threshold? Why is crypto different (other than some belief that it might be more convenient).