Why can’t I boot from the USB 3.0 ports on my PCI add-in card?

The short answer is – you cannot boot via the BIOS on any device that is an ‘add-in’ device, unless it is a PCI card that also contains an option ROM…

Unfortunately, I am not aware of any USB 3.0 add-in cards which have their own Option ROMs that allow you to boot from their USB 3.0 ports.