Yea I dont think theres any market for creating networking hardware that work via USB cables and the Serial protocal. Etherent is used and works because it is so expandable, and there is no language barriers between operating systems and computer types. With USB, the router would need every connected computer to run a driver, and that could be a problem for non windows based operating systems. Thats a bad thing to lay out on a router. And what about adding switches and access points? you cant do that via usb.
So networking via USB does indeed suck.