You can’t directly plug two USB ports into one another, they will both fry each other out (due to the power provided with USB). There is such a technology, IEEE1394 networking, but Firewire allows you to connect multiple computers by design, USB is designed so that you can’t, specifically.
Also, moving onto bandwith, USB 1.1 gets around 10 Mbps (although USB NICs existed that were 10/100, that was for compatibility and not the ability to use the bandwith). USB 2.0 is around 400 Mbps. Both numbers are half duplex. IEEE1394 reaches 400, 800 or 1600 Mbps, half or full duplex and 802.11 (Ethernet) is at 8, 10, 16, 100 Mbps or 1 or 10 Gbps, half or full duplex. FC is 1, 2 or 4 Gbps, full duplex, and SCSI is 640, 1280 and 2560 Mbps. Needless to say, USB is the less robust interface here
There are a few other technical reasons, including packet size and the bandwith sharing, why TCP/IP over USB just doesn’t work. However, you can now pump SCSI over CAT5