The US dollar is "backed" in the sense that if it crashes
No it's backed by something stronger than that. The US dollar is backed by the sheer volume of which it is traded for material goods. It's backed by the sheer volume of its international trade. It's also backed by the treasury bonds and as you eluded to the actions of a governing body that has an interest in keeping it stable.
It's got more than that behind it, too. It's backed by a massive network of debt contracts obligating people to do work in order to generate value in the form of dollars. That's how dollars are created: When banks issue loans they create the money[*] they're lending out at the same instant they create the loan contract that obligates someone to repay it.
Fiat currencies (which BTC is not) are essentially built on the legally-enforceable promise of people to do work in the future. They're backed by human energy and effort.
There are some science fiction stories in which civilizations have found a way to store large amounts of energy safely in small coins, which are used as currency. In those fictional civilizations, when you give someone money you are literally giving them power that they can use to do things. Some fantasy worlds do the same with congealed magic as currency (I recently read the "He Who Fights with Monsters" series, which has a world that does this). The value of such a coin is what you can do with it, completely aside from anyone's opinion about the value, and what you can do with it is defined by the contained energy. That would be the perfect currency, because the coins' worth is 100% intrinsic.
But fiat currencies are almost as good! They aren't encapsulated power to do stuff, but they represent a promise by some person or people to do stuff. They're very nearly 100% intrinsic value (unlike, say, gold, which has a little intrinsic value plus a lot of speculative value).
Bitcoin, on the other hand, is 100% speculative, with nothing at all backing it.
[*] It used to be the case that they arguably only created ~90% of the money they lent, but the reserve requirement was reduced to zero. Even before then it was debatable because while loans "tied up" reserve capital, requiring the banks to hold it in reserve, they did hold it, they didn't lend it out. In any case, with a 0% reserve requirement banks definitely now create every dollar they lend.