That's actually not true right now. Every time you trade it's a taxable event. So if you went USD --> BTC --> ETH --> BTC --> USD you would need to show 4 transactions to the IRS.
The total tax burden will look a lot like (Final USD - Total USD) * (Short Term Cap Gains Rate) but you can't just report it that way.
In 2017 I mucked around with Crypto and accumulated about 50 transactions over ~5 currencies. Reporting was a monster pain in the ass.
Given the libertarian bent of some people who are most attracted to Cryptocurrencies I'm guessing many don't report anything to the IRS voluntarily.
Even regular capital gains are kind of a pain to report, especially when you've inherited the stocks from a deceased relative who kept no records about them.
The total tax burden will look a lot like (Final USD - Total USD) * (Short Term Cap Gains Rate) but you can't just report it that way.
In 2017 I mucked around with Crypto and accumulated about 50 transactions over ~5 currencies. Reporting was a monster pain in the ass.