It's "faster" by design. Legacy serial port (RS232) can move 115200 bits per second, while USB ports can (on theory) move four thousand times more (480 mega-bits per second). That's why we have USB cameras, USB diskts, USB that, and we don't have them connected over RS232.
It really does all depend what you mean by serial..
USB is not faster than Serial ATA. It is a serial interface and has ports including external ones. SATA Revision 3 is 6Gbps which kind of piddles on USB1 and USB2 and is still a whole 1Gbps faster than USB3. In theory there is no reason that interface could not be used to connect things other than discs.