Why would anyone need a library when we could just do this?

    arg=default
    while [ -n "${1}" ]
    do
        case "${1}" in
            --arg)
                shift
                arg="${1}"
                shift
                ;;
            *)
                print_help_page
                shift
                ;;
        esac
    done

The above is terse, readable, and doesn't rely on any dependencies other than POSIX compatibility.

Often the whole point of shell scripts is that they're dependency free; nothing is needed other than the shell. After all, why code rustup in sh if not for this reason?

People who make libraries like this for shell are, I think, missing the point of shell.