As I mentioned in another comment, the linux kernel feature (user namespaces) that enables "rootless" containers was released in February 2013, and Docker was released soon after in March of that year. For whatever reason, they haven't made it a priority to make rootless the default, although it is technically doable. If you are annoyed by this, I'd suggest checking out podman, which has done a lot of work to be basically a drop in replacement with a similar workflow to docker.