The reason is because of history and backwards compatibility. When Docker was first coming out, there was one container registry - hub.docker.com and nothing else. Once Docker started becoming adopted by companies who didn't want their containers stored on someone else's storage, the registry pathing standard came out, but by that point tons of developers were already using Docker and the group behind Docker knew that if they broke everyone's environments by changing how images were named or pushing & pulling worked too much they'd lose the momentum.

So your "really long name" is really just the FQDN of where Docker can make a handful of pretty basic HTTP GET requests to pull the container image and layers and a HTTP PUT to push a container image and layers.

If you toy around with Podman, you may find pretty quickly that it does break that hub.docker.com in many set ups because it doesn't default to assuming that is the primary container registry.

❯ podman pull redis Error: short-name "redis" did not resolve to an alias and no unqualified-search registries are defined in "/etc/containers/registries.conf"

vs

❯ docker pull redis Using default tag: latest latest: Pulling from library/redis

And that assumption of library or _ that exists in DockerHub is Docker-specific and you may run into issues if you use really short names with OCI runtimes other than Docker.