USB is an asymmetric interface. Generally speaking, the protocol of this interface requires a host and a device. The request is initiated by the host, and the device only responds simply. Ethernet and serial ports are symmetric interfaces. The protocol of this interface requires that the devices at both ends are peer-to-peer, requiring that both sides have a fully equivalent protocol stack, and a transport model. To put it bluntly, it is very simple: USB protocol stack on the device side can be very simple, and it is very complicated to implant a complete TCP/IP+ Ethernet driver on the device side. So, we can make a very simple U disk, the device side is very simple based on USB. However, if the U disk is replaced with a network interface, the complexity of the device side will increase and the cost will increase. Therefore, an interface like USB will inevitably exist for a long time. Because of the cost problem, it is impossible to implant a complete network protocol stack for each device. The cost of the network card chip is still much more expensive than the USB module. Moreover, the network is powered by two-way (the specification of POE has not yet been widely popularized), USB only needs to be powered by the host, and it is set at the beginning. In addition, USB speed is not too slow, right? USB3.0 is 5Gbps, USB3.1 is 10Gbps, USB3.0 is similar, but 10Gb network card is not popular enough? Moreover, the speed of SATA3 is only 6Gbps. Even if you configure a 10Gbps network card, the speed of SATA3 write hard disk can't keep up. Even if SATA has no bottleneck, the general mechanical hard disk is difficult to reach very high speed. If you use PCI-e SSD, do you know how expensive is this structure of USB mobile hard disk? There are no manufacturers who are not sure about doing this kind of thing.