For two reasons, both related to the Principle of Least Astonishment: * Because the likelihood that someone entering `echo --help` _actually_ wants to print `--

For two reasons, both related to the Principle of Least Astonishment: * Because the likelihood that someone entering `echo --help` _actually_ wants to print `--help` is staggeringly low * Because it's not the "single solitary string" that has special handling - `echo -n` does not print `-n`. `echo` already supports flags, and so it would be most-consistent and least-surprising for it to also support a standard default one