USB-to-aux cables exist, and they work for the purposes that they were designed. However, they don't work as a conduit for digital music files to your car radio.
Some devices are designed to receive power via 3.5 mm TRS connections, which is one of the main reasons aux-to-USB cables exist.
If you plug a USB thumb drive into a USB-to-aux cable and plug the cable into a head unit, nothing happens. The same is true, in most cases, if you plug a USB-to-aux cable into a phone and connect it to a head unit.