You seem to be misunderstanding the terms "serial cable" and "cross over cable".
A serial cable is a cable where each data bit passes over the same electrical conductor. An Ethernet cable (either straight or cross over), a USB cable, a SATA cable, a Firewire cable, RS232 ports and even the telephone line are all serial cables.
The older types of disk drive using IDE (or PATA) cables, and printers used to be connected with 25 pin connectors are parallel cables - there are multiple data lines which carry different bits of the data at the same time.
When using Ethernet to connect two devices together, you have to ensure that the pin that is sending data from one device connected to the pin that is designed to receive the data at the other device. With a straight cable, pin1 at one end of the cable connects to pin 1 at the other end. Similarly, pin2 connects to pin 2 and so on. To connect two devices together, the socket on one device has to have the pins connected internally in a different order from the other device. The PC ports on a router have their pins in one order, and the port on a computer has its pins in a different order. A straight cable will connect the sending pin on the router to the receiving pin on the computer, and the same in the other direction.
However, if you want to connect one computer to another computer, then a cross over cable is required.
Some routers have a WAN Ethernet port and 4 LAN PC Ethernet ports. The WAN port has its pins organized in the same way as a computer, so that it can connect with a straight cable to the PC port on another router, or to a modem. A cross over cable is required if you connect a PC port on one router to the PC port on another router.
Many modern devices solve this problem of straight versus cross over cables by making the port automatically swap its configuration of pins electronically to match the cable that is in use. With one of these devices, either a straight or a cross over cable will work. With auto-switching Ethernet ports, only one of the two devices connected to a cable needs to be auto-switching.
USB also needs to have a cross over connection. Normally this is hidden from the user as the connectors at each end of the cable are different so it is not apparent that there is any crossover taking place. (Ethernet cables have the same connector at both ends.)
USB and Ethernet are two different technologies. USB2 is faster then 100 Mbps Ethernet, but 1 Gbps Ethernet is faster than USB2. USB3 is faster at 5 Gbps, but not many devices currently have USB3, and not many devices actually need the speed of USB3.
Ethernet is primarily a networking technology. While USB is primarily a point to point technology. With USB, it is designed to be plug and play. You do not need to configure the network as only a limited number of devices can be connected and the connectivity is all handled by the driver levels of the operating system. Ethernet requires addresses and routing tables to be defined so that the network, including its connection to the Internet, all works as needed.
Finally, Ethernet is defined to work over distances of 100 metres, while USB is constrained to distance of 5 metres.