You've misunderstood what currency backing means.
A something-backed currency means that there have to be a _fixed_ amount of physical entity somewhere in the possession of whoever decides to give out the currency. This is not the case with the current fiat. Sure it can be exchanged for all those things you mentioned but it's backed by none of them, some central bankers could agree to create ex nihilo enough money to give a billion USD to every bank account in the world. If you then reason that a dollar is backed by cars then either there would be a huge surplus of cars somewhere to allow this to happen, or someone managed to multiply the current global carpool a millionfold overnight.