As to why it wasn't there initially ... why should it have been? How many people were going to put their desktops close enough together that a 1m USB cable could reach between them?
Peak speed on USB1 (1996) was 12Mbps - home and corporate networks were already operating at at least 10Mbps, and handle data transfer quite nicely (and over far longer distances)
If you wanted to go host-to-host with ethernet, you only needed a crossover cable to do it
USB access for external devices like Zip drives was available - and, if you used Zip disks, the media'd interoperate between internal and external variants
One of the first computers to exclusively rely on USB for peripherals was the first-generation iMac back in 199
USB thumb/external drives appeared about the same time (a couple "history of thumb drive" sites list it as 1999, others as 1998 ... I suspect is the difference is between "design/prototype" and "on sale")
In short, there was no reason to enable host-to-host transfers with USB
> USB was designed to standardize the connection of peripherals to personal computers, both to communicate with and to supply electric power.
> USB has a strict tree network topology and master/slave protocol for addressing peripheral devices; those devices cannot interact with one another except via the host, and two hosts cannot communicate over their USB ports directly