It is important to decode, to understand, “USB.” It stands for “Universal Serial Bus.” “Universal” refers to the standard; thousands of different devices employ the standard for interoperability. It is a standard interface but it is more than that. It attaches to the bus of the host device as peer with processors, memory, and other external input/output devices. The standard provides for the device to contain executable software to facilitate its attachment to and interaction with the host device. Think of it this way; any device attached to the bus is logically an internal, not external, device.
Like many standards, this one is popular, in large part, because it is convenient. It is an open interface for attaching cameras, scanners, printers, speakers, microphones, head sets, monitors, and storage devices. As is often, not to say usually, the case, convenience trumps security. Any control that limited the attachment, i.e., was more secure, would make it less convenient.